1. Boss Fights Sucked.
2. Gargoyles a bit too convenient, grappling mechanic felt too automatic, no finesse involved.
3. Hollow gameplay sections: Courtyard filled with killing dozens and dozens of plant pods (that shoot plasma balls WTF?) by crouching and pressing a button to initiate the canned animation. And hopefully you enjoy that canned animation of him punching into and then ripping open the pod, splattering the fake camera with plant blood, because it's gonna be the same every single fucking time. Which is really boring after the 3rd time. Nevermind the 20th.
4. Gameplay length seems low, if you compare it to some other games released. If you compare it to Modern Warfare 2's length though, it seems pretty great. So that's really hard to say. Coming off of a run of Dragon Age or Borderlands or AC2, it's undoubtedly going to seem a bit short. You really have to judge if the quality of the experience is enough to make up the difference.
There's no real replayability though. It's not linear in the sense that it's a straight line, there are hub areas involved, so you'll have to backtrack and encounter the same areas but with different conditions. But all in all, the campaign's gonna be exactly the same. The only replayability might be in getting higher scores in the Danger Rooms. Wait, Danger Rooms are the rooms in X-Men. What are they called, hazard rooms? Challenge rooms? The challenge is in getting higher scores, but I just played em once each and had enough. It only served to remind me that these sections were taken from the campaign and that there was a lot more to do because you weren't confined inside a room.
5. Other then that, the one big aesthetic detail that I really disliked was what I had been kvetching about for the longest time... before the game, during the game, and now after the game: Batman's walking animation looks retarded. He's just walking like a stiff robot that's maybe a little bit constipated? It just looks bad, man. Seriously, fix that animation. He looks great when he's moving while crouched, but we're all too damn lazy to hold down the crouch button all the time. And when we aren't running, we have to walk. And Batman looks awful doing it. It was a bad jig to begin with, we never should have started this jig. It was a bad jig, a terrible terrible jig.
Arkham Asylum actually didn't have a lot of faults, I enjoyed it quite a bit.