Rocksteady are a pretty new developer and Batman: Arkham Asylum is only their second game, but it feels a game that's from an experienced developer that had the time and the resources to make a truly great game. That's the kind of polish and just plain solidness you expect from big studios that had a few games under their belt.
What I've been wondering is whether the studio's leads are industry known people? That would explain how they got such an amazing second game on time and of such quality.