Okay, I've kinda processed the story a bit more since finishing it, and here's my overall review. I'll try and keep it vague for the spoiler-phobes.
It felt short. Here we are in this world that is literally twice as big as the world in the previous game, but I don't really think they took advantage of that. Most of the missions in the "new section" probably could have been worked into the old section with relative ease. All they'd have to do is convert some of the unused buildings into other things. There are sections of the city that I don't even think I ventured into throughout the whole game.
I was a little disappointed by the different assassins and the Black Mask part of the story (which I was lead to believe was the whole story). I was hoping to get to know some new characters, but instead they all played like random bosses. The gameplay was fine, but the story left me wanting.
Overall, I liked the game (and I still have a lot of side missions to do). It definitely felt like a part of the Arkham continuity, and the gameplay was good, but for a game that was literally twice as big as its predecessor, it somehow seemed smaller.
If I had to grade it now, I'd probably give it a B. I'm only at 25% completion, so obviously there is a lot more to accomplish. I wish that games like this would do a better job of including the side missions in the main story. I shouldn't be able to finish the story and still have 3/4 of the game left to play!