I think it feels kind of short because it backloaded with the plot. I mean for the first part of the game it doesn't seem a whole lot is going on and then about halfway through or so, it really kicks in. All of the interaction and development of Gordon and Alfred is really in the latter portions of the game.
I don't really understand the previous poster who stated that there were no references to Arkham in the game. Maybe it's because I explored more and didn't rush through the main plot, but I saw all kinds of references to the Asylum. Not even counting the conversation during the credits that specifically deals with the issue. Heck, one of the characters even winds up in what looks like the Asylum at the end.
And, I'd like to add that Troy Baker does an excellent job as The Joker. I personally never felt that he was doing an impersonation of Mark Hamill. IMHO, he seems to have a darker, more menacing quality to his delivery than Hamill. Not necessarily BETTER, but different.