Heath Ledger as the Joker was--as everyone has declared--a revelation, although I think the great Gary Oldman did just as amazing of a job. If not better. Christian Bale was trickier for me this time around. Maybe it's because he had to share the screen time with so many other people, but I thought his Batman performance was weaker than in the previous film. His Bruce Wayne, on the other hand, was sensational.
Unfortunately, I thought Maggie Gyllenhaal, whom I love in most films, gave just as weak of a performance as Katie Holmes. Maybe it's just the way they write that character, but I felt very little from her. Aaron Eckhart did his usual competent work as Harvey Dent.
ETA: Oh, if Nestor Carbonell is mayor in the next movie, could we give him less eye makeup? Dude's eyes cannot really look like that.