Rocksteady's Arkham Asylum was one of those games that had a lot to prove and more than managed it. Superhero games generally, and certainly Batman games (Telltales' Lego Batman aside) have a deservedly poor reputation, and a game that not only managed to simply good, but also capture the world and feel of being the Caped Crusader in a focused, kinetic and downright fun game.
So the inevitable sequel arrives, promising more. More villains, more story, more locations. Holy Feature Creep, Batman! Sequels generally go in one of two directions - make the same game again (hello Uncharted 3!) or make large-scale changes and risk breaking a successful formula. Arkham City attempts to split the difference, bring some pretty big changes on the one hand, and staying true to a successful formula on the other.