For the boss fights, there is a complete lack of variety.
Yes, I enjoyed the battles with Bane, Scarecrow and Poison Ivy (Killer Croc was kind of lame), but what really disappointed me was how often I had to battle stupid Titan mutants, right up to the climax! Every major battle from that point on involved fighting one or two titan mutants and a handful of thugs. The game taunts you with Character Bios, which include Two-Face and the Riddler, and then only gives you enough villains to count on one hand. On top of all this, you pretty much use the same "batarang-throwing" tactics for each boss with the exception of Scarecrow.
The biggest disappointment for me though was the final battle with Joker, if you can call it a battle... He barely even fights you! He just swings his arms around for a couple seconds and then jumps up onto a platform waving at the Gotham helicopters' cameras. He becomes completely oblivious to the fact that you are standing right behind him. Even if you grapple him in the back and bring him down a couple times, He continues to get right back up and remain oblivious to your presence. The Joker is a lunatic, but he's not that stupid.
Batman Arkham Asylum had everything going for it: great combat, great level designs, great riddles, great graphics, and a superb soundtrack, but I can't help but feel a little gypped by the lack a variety concerning the boss fights.
--Aaron Renfroe