Just going to throw my opinion in there about it.
Realize this only reflects my opinion, and given that my opinion is completely subjective and not based on anything of actual substance (which from what I can tell is everyone's opinion), I suggest you take it with enough salt to kill a baby.
I don't really see why this game is so popular. It's kind of like what would happen if a 12 year old savant with decent programming skills and an obsession with poop and immature things made a bullet hell and roguelike hybrid.
I mean I can see the appeal, if you're into that sort of thing, I guess the size of the audience just surprises me! It's not a bad thing at all certainly, to each his own (it's not like I have any better reasons for the games I happen to like). It's certainly one of the most streamed games on Twitch.
I just don't get it. I think there's just a lack of interesting bullet hells out there that are either A) Way too generic (the overdone Japanese type) B) Way too difficult (even on "easy") and C) With little or no replayability after you beat them.
So even for all of BOI's flaws, it is kind of unique in its own way, normal mode does a good job gradually introduces you to the game, and I suppose that if you don't care about the game's random nature, it has a lot of replay value.
I do wish the game had a better multiplayer feature (the local co op thing is a joke), it would appeal to me more if I could play it with others.
One final thing, I don't agree with the people saying that the random nature/drops of the game are often or even sometimes an instant loss. I see streamers all the time who have 300+ runs in a row on the hardest difficult without having lost one. If you get good enough, you can work around the random factor, just takes more practice.
Well, a couple of things: Firstly, I dont think the theme/graphics/poo is meant to be at all part of the appeal... it's just Edmund's style. If you've seen his other games, they tend to be equally gross and weird. I personally dont really mind it. In order to fit the lore and ideas behind the story, making it downright disturbing DOES make sense. Besides, I do like that it never really takes itself too seriously. I'm tired of so many "mature" games these days obsessing over that.
And second, it's not meant to be a bullet-hell game to begin with. Which is good, because it doesnt come close to being one. And I definitely am in a position to judge that aspect. The combat tends to be alot more "slow and deliberate", even though there's alot of dodging. A bit like how the very first Zelda game was (later games simply lack this aspect entirely).
Alot of the appeal comes from a couple of things:
1. Randomness. This is what makes any roguelike what it is, and is the core draw of the genre. But with this game, it's a bit more than that. It allows every run to actually feel DIFFERENT than the last. Very different. This is actually a very, very difficult thing to manage in the design of any roguelike or similar game; most of them, as in damn near all of them, simply cant do it. But this one does it perfectly. This very dramatically increases the replay value, and roguelikes in general tend to have a ton of this just by default.
2. Decision-making. Like you said, the random nature of the game isnt the cause for losses. It just isnt. For ANY good roguelike of any sort, this needs to be true, or it isnt a very good game. In this one, skill, knowledge, and thinking will always trump the RNG. Aside from the actual combat, it's all about making decisions, and there's ALOT of decisions to be made, and a ton of complex factors go into it. I'm not going to describe that here; I already rambled about that in the earliest part of this thread. But to me, this is another super-important aspect of this genre, and this one not only gets it right, but offers a HUGE number of possible situations that you never know what sort of difficult decision it might throw at you next.
3. Extreme variety. Some 500 or so items now (and I mean "power items", like the sorts of really major items that you find in a dungeon in Zelda), some equally unholy number of different enemy types, 70 bazillion bosses, and a *really* absurd number of possible rooms to encounter (the expansion alone adds over 1000 new rooms). Aside from extremely specific bosses such as Mom or Satan or whatever that appear in specific spots every time, you never know just what the game will throw at you next. This all interacts alot with the previous two points.
4. It's just fun. Too many developers forget these days that games are, at their core, supposed to be fun. This one, though, doesnt forget it even for a second. It's fun to the point where, even if you have a really stupid loss, you still want to play again anyway.
Now as for the bullet hell games.... I do agree on the "generic" aspect for alot of them. I blame Touhou. It got too popular, and it basically suffers from the "Warcraft Effect", where anyone interested in making a bullet-hell shmup sees the success, decides "I'll have success if I do it too! OBVIOUSLY! How could this not be true?", and thus, copies it. So, they all look the same for the most part. As for the other two points though.... if these games are easy, they get VERY boring. I just cant even do the easy ones. They'd put me to sleep. That being said, there ARE easy ones out there. If you want a good entry point into the genre to try out, try Giga Wing, which can be found on Mame. I dont really play that anymore myself.... it's just too easy for me now.... but it was indeed my initial foray into this genre, and it IS a good game. As for replayability... these games arent designed to just be "beaten". Provided you ever do beat them. They're usually designed, from the ground up, to be played for score. Usually these scoring systems are pretty complex, to the point where some of them are genuinely hard to explain. I, frankly, wouldnt be so interested in the genre if it wasnt for this aspect. The idea of getting better and better, achieving higher and higher scores, and seeing your rank on the leaderboards rise IS the replay value. Alot like old arcade games, and this genre DID start in arcades.
And I'll agree on Isaac's multiplayer bit. Seems dumb to me. Though, roguelikes often dont mesh well with the idea of multiplayer to begin with.