The Binding of Isaac : Rebirth seems to be a wonderful remake of the original. It’s got a great engine, smooth animations, new items, new synergies and even a new level — what’s not to like? Well, if you’re new to Isaac, you’ll probably miss a lot of the issues that I’m about to discuss. But if you’ve played the original (‘vanilla’, as it’s commonly called), after the excitement dies down, you’ll find that there’s a problem with the game — it’s just too easy. It won’t be clear why that is — everything seems faithful to the original’s design. But what you’ll experience is run after run of steamrolling through the game. A run in Rebirth starts out challenging, much like vanilla, and then quickly becomes both interesting, since you’ll become overly powerful, and boring, since you’ll be so powerful nothing in the game will challenge you. Chances are, you’ll plow through every enemy the game has to offer past level 3 with little threat to yourself. Notice that I’m not saying that it happens all the time — there will be some weak runs. But I estimate that around 70 to 80% of your runs will be overpowered.
You might think that your success at the game is simply a function of you being familiar with it. This is not the case. Go play a run of vanilla Isaac (without using the D6, which I’ll get to later). You’ll probably feel challenged throughout the run. In other words, you’re not that good — the game is just much easier (on average).
Why is this? What elements of design make Rebirth so much easier and OP-inducing than the original? Let’s examine the different elements of the remake’s design and see what we can find.
Item Chances and Devil Deals
Isaac is all about randomness and getting items, right? Well, the main place where that happens in a fair way is through the item room. Vanilla Isaac with WoTL had around 200 items, and Rebirth has around 300. Most of these items sit in the item room pool. Some items are very strong, while others are weak. Some are situational (Razor Blade), while others are all-around fantastic (Epic Fetus). But the most important thing is that there are a lot of items, and if you rank them into weak, good, strong, and game-winning, as I do here, you’ll find that there’s a fairly nice ratio there, especially in vanilla. Let’s say that there are 4 game-winning (legendary) items, 30 strong (very rare/rare) items, 100 good (uncommon/common) items, and 25 weak items. This is a good ratio, because it means that you’re not always going to get the strongest items in the game. Once in a while you’ll get lucky, and that’s good — the occasional strong item will give you a needed boost, but hopefully not immediately win the game for you. Once in a while, you should get a really game-winning item and become OP, and that’s ok — Isaac is a short game, so becoming OP once in a while isn’t a huge deal and is fun. But you don’t want to be OP all the time. That’s just boring.
Now what about the devil room items? The devil items are much fewer than the item pool — there are only 40 of them (in vanilla there were 30), and a very high percentage of them is good. There are 3 legendary items, 13 rare/very rare items, 12 uncommon/common items, and only 2 weak items. This means that the chance of getting OP through the devil deals is very high. In fact, I identify the devil pool as the weakest point of the Isaac design (for both Rebirth and vanilla) since it is almost always the dependable way to become OP in the game. Get a few devil deals, and your chances of beating the game become significantly higher. This means that the strategy to success in Isaac almost always involves getting devil deals, while in order to keep player power in check and remain challenging, the game needs to limit your access to the devil pool. Otherwise, the player’s runs will be both consistently OP and homogenous (dominated by the same few devil items).
Interestingly, while vanilla Isaac had a smaller devil pool (30 items), that pool was much weaker on average than Rebirth’s devil pool (no Lil’ Brimstone, Dark Bum, Abaddon, Satanic Bible, Gimpy, Contract From Below, Goat Head etc). So even though Rebirth ameliorated the problem of Brimstone and Mom’s Knife being too common by making the pool bigger, it made the average Devil deal items much stronger, exacerbating the problem in that sense.
It’s interesting to keep going with the comparison: Rebirth has a 1/7 chance of offering you 3 devil deals, while vanilla only had a maximum of 2. Obviously, this makes every deal more capable of making the player OP in rebirth. To balance this out, however, the devs introduced a 1/7 chance of getting only 3 red chests in a devil deal. As we’ll see below, this ‘bad’ devil deal isn’t really bad at all. On the other hand, vanilla’s Curse Room had a chance of spawning devil room or angel room items, meaning that you could get a free devil deal. This increased the chance of seeing Brimstone or Mom’s Knife in a game. Rebirth, however, changed it so that the Curse Room had a chance of spawning out of its own pool, which contains all the Guppy items as well as Goat Head. For the impact of this decision, keep reading.
While some of the different design decisions in vanilla and Rebirth balance each other, the takeaway from this section is that Rebirth has less of a chance of spawning Brimstone and Mom’s Knife (which was a complaint with vanilla) but has a much stronger Devil Deal on average. But this by itself clearly isn’t the source of the easiness of Rebirth.
Spirit Heart Rarity
Consider the mechanism of getting Devil deals: it’s all about avoiding red heart damage. One way to do this is to avoid all damage, which is hard, and is a real measure of skill. The easier way to do it is to get spirit hearts to serve as a buffer for your red hearts. This makes spirit hearts immensely valuable – much more so than red hearts – since they are the tickets to getting into devil rooms, which are the main way to get strong.
Spirit hearts have another value to them – they are the only hearts you’ll build up if you use your red hearts as devil currency. If the devil deals function as intended, you should be bartering your health for strength, risking yourself in the process — a true ‘deal with the devil’. However, if spirit hearts are easy to collect, then that doesn’t mean anything — sure, you lost some red hearts, but you can just replace them with spirit hearts (and as a bonus, get more devil deals).
What this all amounts to is that for the sake of making the devil deals work as a concept, spirit hearts need to be rare. They can’t appear all the time, as that would compromise the mechanics of the Devil room.
Were spirit hearts rare in vanilla? I would say that in general, they were. However, almost all easily OP runs in vanilla that didn’t originate in being lucky with item-room items, came out of some ability to get more spirit hearts than the game’s design was built for. The sources of spirit hearts in vanilla were random drops (small chance of a spirit hearts), tinted rocks in the first few levels, Guppy’s Paw, the Book of Revelations, the Relic, red chests, Mom’s Pearl, the Hierophant card, and the Fortune Telling Machine. It’s no coincidence that the Book of Revelations and the Relic were two of the strongest items in vanilla, since they were such a consistent source of spirit hearts. However, they were also quite rare. Guppy’s Paw, however, was highly exploitable, since it gave you a very high 3 to 1 ratio of spirit hearts to red hearts. The Fortune Teller Machine was also a very weak design point in my opinion, allowing you to convert money (a plentiful resource) to both trinkets and spirit hearts. However, aside from these particular sources, spirit hearts were indeed rare in vanilla, making it very hard to play with blue baby as your character, for example.
Now let’s examine the commonness of spirit hearts in Rebirth. Rebirth contains every source of spirit hearts from vanilla, and many more. Tinted rocks are much more common in Rebirth, persisting all throughout the game. Additionally, many Rebirth maps contain blue or pink fires which can be put out with bombs. These have a high chance of giving you spirit hearts, and often are situated in clumps that are easily bombed together using just one bomb. Rebirth also contains more items that give you spirit hearts, such as Gimpy and Dark Bum, for example, and the upgraded shop concept in Rebirth – about which I’ll talk below – means that there will almost always be a spirit heart for sale.
But Rebirth also contains an entirely new type of heart called a black, or devil heart. These hearts count like spirit hearts and have all their associated issues, but add a new problem to the mix: when you lose a black heart, you trigger the Necronomicon effect. This means every enemy on-screen gets hit for 40 points of damage. While that may not seem so bad, the impact on Isaac’s design is devastating: the majority of enemies in Isaac have less than 40 points of health, and most damage in Isaac happens not from giant bosses – whose patterns you learn over time, since you face them repeatedly – but from small enemies who slowly whittle down your health room after room. Black hearts ensure that situations that would have challenged you in vanilla – where the player takes damage – result in just about every enemy on-screen getting wiped out immediately. The effect is subtle but builds up over time.
Once again, this wouldn’t be so bad if black hearts, together with spirit hearts, were rare. Unfortunately they aren’t. The super secret room often contains a black heart. The middle levels of the game contain skulls, which often give out black hearts. The devil room itself often contains a black heart and sometimes even gives you 2 black hearts + a deal, making the deal effectively cost only 1 spirit heart. Many devil room items also give you black hearts, including the Satanic Bible — an item stronger than the Book of Revelations, since it deals out black hearts, and Abaddon, which removes all your red hearts and gives you 6 black hearts. Finally, Curse Rooms often contain several black hearts.
All of which brings us to the following point: spirit hearts, and their more problematic black heart cousins, end up being far more common in Rebirth, and this greatly skews the percentage of obtained devil deals, as spirit/black hearts are the keys to obtaining devil deals. A pattern that emerges is that Rebirth buffs the player’s strength, while keeping the challenges of the game around the same level as vanilla, thus making the game too easy.
Free Devil Deals
Rebirth contains a concept that was very rare in vanilla (with the exception of the rare joker card): the existence of items that give you guaranteed access to every devil deal in the game. The Book of Belial is one such item, but I consider it the lesser offender. Not only is it relatively rare, it also competes for your active item slot, which means that if you want some other active effect, you’ll have to ditch it. Nevertheless, it’s not clear why the buff to this item was necessary. In vanilla, the Book of Belial increased your chances of getting a devil deal, but didn’t make it a sure thing.
The worse item in my opinion is Goat Head, which passively grants you every devil deal in the game up to the Cathedral/Sheol. Considering the strength of devil deals and their ability to influence and skew the game, this is a very big design issue. Additionally, the Goat Head was stuck in just about every small pool: it’s found in the devil item pool itself, in the red chest pool, in the curse room pool, and in the golden chest pool. The result is that a player has a very high chance of getting an item that completely eliminates any barriers to obtaining the strongest items in the game. Now of course in some runs, the player could get this item after it loses its relevance, but on average, this item will make a huge impact on the game’s balance. This item would have probably been ok had it been stuck in the general item pool where it would turn up rarely, mixing up the gameplay (as all good items do), but making it as common as it is was a very problematic decision.
Blue Flies, Blue Spiders
Blue flies (or ‘attack flies’) were, in my opinion, one of the worst designs in vanilla Isaac. These are flies granted to you by different sources. They orbit your character until they find an enemy, at which point they attack the enemy and expire on the spot. The problematic point with these flies is that they do twice your regular damage. This means that they force upon you bland gameplay, where enemies expire instantaneously. With decent damage and a few flies, you could enter a room and immediately wipe out its enemies.
This design decision was very strange. Why do the blue files scale with your damage? Almost everything that does a lot of damage in Isaac is big and powerful-looking, and yet these innocent-seeming, tiny flies do more damage than even your tears. It would have made so much more sense to keep them at a constant level of damage or at least to cap their damage.
Regardless, blue flies were mitigated by 2 factors in vanilla: first, they were quite rare. You could obtain them randomly from chests, or you could get them from Infestation when hit, from Guppy’s Head when used, or from Mulligan (and Guppy) when you hit enemies. Of these sources, Mulligan and Guppy were the worst, because every hit spawns a fly. If you had an attack that triggered many ticks of damage, like Brimstone, you could have very many flies indeed. But then, the second mitigating factor kicked in: because of Flash’s limitations, you could only carry 5-6 flies between rooms. This meant that you couldn’t just march in with an army of flies and demolish enemies instantly. So, as bad as this design was, it was limited in its severity.
Rebirth makes the fly problem much, much worse. First, it introduces a new kind of blue ally: the blue spider. Because the spider can’t fly, and therefore can’t reach your enemies as easily, it was given even more damage, to the tune of 2.5x your damage level. Aside from this, though, there are now many more items that produce blue flies and blue spiders at varying rates: Rotten Baby, Mom’s Wig, Box of Spiders, Infestation 2, Juicy Sack, Sissy Longlegs and Spiderbaby, in addition to the Mulligan, Infestation and Guppy from vanilla. Finally, to show off the new engine’s prowess, all limits on the number of spiders/flies carried from room to room have been lifted. This means you can fairly easily develop huge armies of flies and spiders in easier rooms, and then march them from room to room, where they proceed to demolish everything on the spot. The most OP runs in Rebirth are usually caused by this design issue, which obviates virtually all of the gameplay and skill in the game.
‘Bad’ Devil Deals and Guppy
We’ve mentioned the ‘bad’ Devil deals, which occur 1/7th of the time in Rebirth, and which were almost certainly added as a small ‘nerf’ to Devil Deals, given the fact that they are far more common in Rebirth. Now let’s see why these Devil deals aren’t so bad at all.
Guppy form was added to the Isaac design in WotL, as an alternative path in which to play the game. Rather than just opening golden chests and getting regular items, a player can venture into Curse Rooms at the cost of a heart’s worth of damage. The results of this action can either be negative – fighting several spiders – or very positive, in the form of spirit hearts, an item from the Devil/Angel room, or the elusive red chests, which have a chance of containing either enemy spiders, spirit hearts or Guppy items. The whole thing was supposed to be a high risk/high reward addition to the regular gameplay.
While Guppy items were good, they weren’t amazing, for the most part, on their own. The ultimate reward came from obtaining Guppy form after finding 3 Guppy items, which gave you both flying (which is very powerful) and the Mulligan effect (see above). Due to the blue fly issue described above, Guppy form in Rebirth is an instant won run, making you essentially invulnerable.
Another problem with the implementation of Guppy is that it’s just a numbers game. The red chest pool contains almost exclusively Guppy items. As such, the probabilities are easy to calculate. After opening about 20 red chests, you have a bigger chance of being Guppy than not being Guppy. The more red chests you open, the closer you arrive at being Guppy and thus of being unbeatable.
Vanilla Isaac gave you a maximum of 2 red chests in a curse room, plus an occasional red chest here and there. The remaining source of Guppy items was the Devil room and the Curse room itself, but without the D6, it wasn’t easy to get Guppy form. Rebirth gives you up to 3 red chests in the Curse room, the occasional red chest, and 1/7 chance of getting 3 red chests in the Devil room. Additionally, there are more Guppy items in Rebirth, making it more common in the Devil deals. And since the Curse room now has its own pool, much of which consists of the Guppy items, and Guppy’s Head even features in the small Golden Chest pool, the result is that Guppy form (and its resulting power) is far more common in Rebirth.
Mushrooms and Skulls, Pills, Cards and Runes
Rebirth has a new destructible terrain type, which in the mid-levels manifests as mushrooms and skulls. These destructible items can generate pills and item; and cards and black hearts, respectively. Seemingly, they were placed there to give some extra strategic options to weak runs, which is good in my opinion, but they do so at the price of flooding the game with resources.
In vanilla, you rarely had too many cards or pills, and thus these potentially game-altering resources were rare and therefore precious. In Rebirth, you have so many cards and pills, you don’t know what to do with them, especially if you’re already tending OP and can therefore use your bombs or stomp skill (from Leo or Thunder Thighs) to break all the containers. This element of Rebirth therefore serves as a huge feedback loop, making OP players who can afford to destroy these items even more OP.
What kinds of bonuses can you get from pills? Tear upgrades, full health and bonus spirit hearts are the most game-altering pills. Cards can give you bonus spirit hearts, doubled resources (bombs, keys or hearts), a free devil room deal, access to the secret room, or just free teleportation, which in the case of Rebirth’s boss rush results in a free item out of a choice of 4. Another new, overly powerful card is 48-hour energy, which generates 3 batteries (see below). Additionally, Rebirth contains powerful runes which the game treats like cards, some of which have extremely strong effects, such as the ability to make you invulnerable for long periods of time, the ability to see all secrets on the map (including super-secret rooms), the ability to double resources (which is best used to double the number of golden chests on the Chest), and so on. The key point is that these resources weren’t available in vanilla — certainly in nowhere near the same quantities.
In vanilla, the shops were randomly sized, from 2 items up to 5. This increased the challenge of the game and the excitement of finding a nice, big shop. In Rebirth, by upgrading your shops over time, you can consistently make 6 items appear. This means that the shops always store a huge variety of items, allowing for such strong items as spirit hearts and lil’ batteries to be carried reliably. Once again, we see the feedback effect at work: strengthening one thing strengthens others, giving the player more and more advantages and buffs.
Edmund himself has stated that he wanted to make the shops more strategic and important, and I get that. There are more powerful items in the store, such as the Broken Watch and the Sharp Plug. But buffing the store without compensating for it elsewhere in the design, in addition to every other buff in the game, places too much strain on the original Isaac design.
As if the large shop size wasn’t enough, each shop contains a donation machine. This machine can be blown up repeatedly, giving you plenty of coins with which to buy whatever you want. It completely eliminates the scarcity of money from the game, so long as you have any bombs or bomb-like powers in your possession. Find something you like but can’t afford? No problem — just bomb the donation machine.
Rebirth includes a new consumable in the form of the lil’ battery. Unfortunately, this consumable completely disregards the number of bars needed to charge items, and charges each item completely. It’s made even worse by the fact that a 48-hour energy card exists, giving you 3 battery consumables on the spot. This card is also relatively common due to the abundance of skulls in the game. These consumables completely upset the balance of active items, even more so than the regular shop charge items such as the Habit or Sharp Plug.
Here is what Edmund had to say about the battery consumables on the Rebirth blog: “the lil battery is a new rare pickup item that only appears if you are holding a usable item, recharging it fully… who likes breaking the game!? (this ones for you!)”. Bypassing the charge time of items completely is in fact very effective in breaking the game, but items such as Satanic Bible and the Book of Revelations become much stronger when they can reliably produce several hearts at a time. Given the large shop sizes, the lil’ batteries unfortunately tend to be quite common.
Flying was always very powerful in Isaac. Many of the traps and difficulties of the mid-game (for example, creep and spikes) only apply when you walk on the ground, or arise from the challenge of using the limited amount of space given to you. Flying is even more effective in Rebirth, where there are shooting traps that can be bypassed and larger pits that can be flown over to completely avoid enemies.
It could easily be argued that vanilla gave out flying far too often. You have 2 devil room flying items, 2 angel room flying items, 1 golden chest flying item (Fate), and 1 regular pool flying item (Transcendence). Given how powerful it is, and given the fact that there are other, more interesting choices to get around obstacles (namely, the ladder and the Pony items), it’s unfortunate that flying is as common as it is.
Rebirth added yet another interesting way of getting around: the ‘How To Jump’ book, which allows you to strategically hop across chasms so long as this active item is in your possession. How unfortunate, then, that Rebirth didn’t do anything to reduce the commonness of flying. Particularly, while Fate was rare in vanilla (with a 50% chance of not being in the pool altogether), it’s always in the golden chest pool in Rebirth with a 1/9 chance. And let us not forget about Guppy form, which is statistically far more common in Rebirth as we discussed above.
I’ve already mentioned some problematic items in passing, but in this section I’ll discuss some more item designs in the game, both good and bad. A good rule of thumb (in my opinion) is that it’s ok for an item to be OP, so long as it’s rare — otherwise the whole balance of the game is upset. Ideally though, items should change the way you play, making things more interesting without making the player too overpowered, and the game could afford to have these kinds of items be more common. In general, it appears like Rebirth had a few really good ideas for items, and then, to reach a large enough number, many items were thrown in without enough consideration of their impact on gameplay. Furthermore, many items become problematic only after synergizing with other items. If the combination is rare, that’s fine, but oftentimes it isn’t.
- Stopwatch is a horrible item, no matter what it’s gated behind (in this case, an exercise in patience). It trivializes the game more than any other item, completely removing any challenge. Additionally, Rebirth implements the slowdown effect of both Stopwatch and Broken Watch very strangely, wrecking the behavior of many enemies in the process.
- Brimstone was buffed up unnecessarily from vanilla. In vanilla, it was an item you could sometimes still lose with, giving you unlimited range at the price of a long, inflexible charge time. The item also didn’t fire for very long unless you abused a bug in its implementation (known as ‘brimsnapping’). In Rebirth, you can decrease the charge-up time of brimstone (and all other charge items) significantly with tears-up pills, essentially removing the element that was critical to brimstone’s balance. Additionally, the brimstone laser is thicker, does more damage, and lasts for longer, making it essentially an instant-win item. Finally, the developers decided that brimstone should also synergize with many items, such as the very common (since it’s in the tiny golden chest pool) Tammy’s Head to produce a room-by-room death machine, or with Bent Spoon to automatically seek out enemies. This means that with one extra item, Brimstone goes from a sure-win to extremely OP.
- Tammy’s Head was a fun item in vanilla, but with its one-room charge and its insane synergy with strong items like Brimstone (and others), it easily becomes an instant (and common) run-winner in Rebirth. This item needs an adjustment to lengthen its cooldown the stronger it gets.
- Gnawed Leaf is a horribly designed item, encouraging the worst kind of gameplay — passively sitting and doing nothing, while your familiars do everything.
- Magic fingers is another lazy item. It scales with your damage, meaning that you can destroy every enemy without even touching it by just using your item, so long as you have money. Money is extremely easy to obtain in Rebirth given the existence of many other money-producing items and trinkets.
- Pyromaniac completely breaks the game, since it both protects you from explosions and heals you.
- Holy mantle also breaks much of the content of the game. Most of the rooms in Isaac whittle you down a little bit at a time — especially because the plentiful black hearts don’t give most monsters more than one chance to attack you. Nullifying that one hit negates the enemies’ main method of attack.
All the factors listed above still don’t explain fully why Rebirth is so much easier than vanilla on average. For a long time, I just couldn’t figured it out. However, it turned out that I was overlooking one crucial detail, which I just happened to gloss over: Special Items. Special items are select items that work based on a different spawning mechanism than all other items in the game. Rather than rolling independently, obtaining (or, in the case of Rebirth, simply observing) a special items causes a counter to go up. The higher the counter, the more times the game re-rolls when producing a new special item. Thus, encountering special items causes fewer and fewer other special items to spawn. As it turns out, this is vanilla Isaac’s main check on obtaining too many OP items.
Is this a good implementation? Well, let’s consider the alternative. A more straightforward approach would assign a probability to each type of item, classified by strength. Legendary items would be extremely rare, and common items would be common, etc. What are the advantages of each approach? The ‘special item’ approach allows the player to get even rare items almost every run. However, you could only ever get a few of these items. It also lumps all special items together into one pool of progressively rare items. The probability approach doesn’t guarantee any rare items — you just have to get lucky. But it doesn’t penalize you for getting a good item. You could get lucky enough to get a whole bunch of great items. Personally, I think the Special Item system was badly designed, and a good probability system would have been much better. Nevertheless, it is the primary way that Isaac prevents you from getting a whole bunch of overly powerful items and steamrolling the game.
Indeed, this is where the biggest difference between Rebirth and vanilla lies. In vanilla, all the items that are rare or above in my google docs sheet – even damage ups – use the Special Item system. This means that all items that are of moderately powerful (rare) level or higher are regulated. Rebirth’s developers didn’t like the Special Item system, and I can’t blame them for that — I happen to agree that it’s far from ideal. But they didn’t replace the old system with anything else. Instead, only the strongest (Legendary) items use the Special Item system. This means that most of the items in the game don’t have any of the probability regulation of vanilla Isaac, and that it’s extremely easy to just keep rolling powerful items far more often than in vanilla. This is why Rebirth makes you so OP. The game has lost its main method of controlling player power, and no other system has been used as a replacement. As a result, Rebirth is easier.
“Just Don’t Take It”
As an aside, a common refrain of defenders of Rebirth is, “If you don’t like an OP item, don’t take it”. I think this argument completely misses much of the point of the game. Isaac is a game mostly about skill: dodging, shooting, etc. But it’s also a game of strategy. Getting a devil deal by shielding your red hearts, heading for the boss early to get a devil deal, blowing a hole from secret rooms to other special rooms — all of these are examples of the strategies that people develop as they learn the game. Much of the strategy comes from knowing how to best use your resources to beat the game. This is part of the reason why the game needs to be challenging — if it isn’t challenging, you don’t need to work too hard to figure out ways to beat it.
Now, if you tell a player not to take an item because it’s OP, you’re essentially telling him to ditch the strategic element of the game. At that point, the game ceases to be a struggle for survival. How much should the player continue to weaken himself to let the game challenge him? It becomes similar to playing a game of chess against a 6 year old — much of the satisfaction is lost. If you lose, is it because you were unskilled, or because you overly handicapped yourself?
Vanilla already suffered from the commonness of Brimstone and Mom’s Knife in devil deals. Many players (including myself) compromised their gameplay by not picking these items even when they were offered them. However, at least these choices were isolated. In Rebirth, you would have to avoid taking almost any item the game gives you to maintain a sense of challenge.
You don’t need this analysis to feel that something is wrong in Rebirth. In fact, I didn’t start out by making this analysis — I started by just feeling something was off. The first 2 levels of Rebirth are very similar in difficulty to vanilla (despite some weaker enemies being mixed in). But then, the problematic design choices of Rebirth start kicking in – whether it’s the plentiful cards/pills, the black hearts, the countless devil deals, the blue flies/spiders or, more than anything, the reduction in Special Items – the result is that the enemies in the later game, which were so meticulously crafted to be challenging in vanilla, become easy cannon fodder to be blown away in Rebirth. The normative situation in Rebirth is that you become OP and obliterate the game starting around level 3 (or a little later). This means that it doesn’t matter how much design effort went into those later levels or enemies — it’s almost all wasted, due to weak balance decisions.
What About Vanilla?
You might claim that vanilla wasn’t very well balanced as well — that you easily became OP in that game. I think most advanced players view that game through D6-tinted glasses. Yes, if you happened to get Mom’s Knife or Brimstone, the game would be easy. And if you used the D6, the game would be easy as well. This can be considered vanilla’s biggest flaw: one of the strongest items in the game also happens to be Isaac’s default item. But assuming I’m talking to a good Isaac player (why else would you be reading this?), if you go play a run of vanilla Isaac right now, odds are you won’t beat the game easily, even if you do get some strong items, so long as you don’t use the D6. Vanilla Isaac had design issues, but they were isolated and limited. And most importantly, it regulated player power via the Special Item mechanism.
The Bottom Line
In my opinion, these design flaws are what causes Rebirth to give you one OP run after another. The game starts out difficult, which lets players feel like they’re challenged, and then, due to a missing regulating factor (Special Items), feedback loops and other balance issues, dramatically eases up on the difficulty, by constantly boosting the player and leaving the enemies at the same difficulty level. This makes even new players feel empowered — they think they’ve gotten great at the game, while in fact, the game just makes them OP very often.
At the end of the day, the points above just reinforce my impression of Rebirth as an easy game ‘pretending’ to be hard (specifically, as hard as its predecessor), while vanilla Isaac was a flawed, yet brilliantly crafted, hard game that sometimes threw you a bone.
BTW, if you agree with my analysis, you may be interested in my mod for Rebirth, where I try to make the game far more balanced and keep the challenge all the way through.