This was by far my favorite super hero movie. I'd actually give it a 5 out of 5 stars, and I am a very harsh movie critic, especially of "blockbusters". (I'm quite the snob, actually)
Firstly, because the writing for the joker was excellent and Heath Ledger pulled off the character as I never hoped. He was convincingly crazy. Not typical hollywood CUCKOO CUCKOO crazy, or I-hurt-people-because-I'm-evil crazy. He seemed like a legitimate psychopath. Heath Ledger managed to portray the Joker as when he was at his absolute best (ex: the killing joke). I was not a fan of Heath Ledger's but after seeing this movie I find myself actually a little saddened he can't act in the future. His Joker is possibly, in my opinion, the greatest portrayal of a psychopathic villain in a film (of course, temper this by the fact that I was the movie about 18 hours ago... let's see if it stands the test of time).
Probably my favorite aspect of his portrayal was how unfunny I thought he was. He was saying dialogue that was written humourous, but his inflections and facial expressions made it the opposite. I was a little peeved at people laughing at every single joke - not sure if I'm being an elitist but I got the feeling they were giving the cheap laugh to something that wasn't made to be funny. It was the complexity of the dialogue and performance that these were funny jokes, told in an unfunny manner. I think some people didn't get that. (EDIT - yeah, I do sound like an elitist).
Secondly, Christian Bale continued to impress showing Batman as he was written by Frank Miller, at his best - truly dark and brooding, but a hero.
The plot moved at a good clip, the action scenes were entertaining, and the small touches really made it work (come on... the diversion was a fire truck--- on fire! perfect joker!). The jokes were a bit too much, but they were honestly funny.
I would complain that at many times the dialogue was more than a little ham-handed with two-face pretty much narrating his downfall.
I can't wait to buy it on DVD and decide if this movie can crack my top 20 all timers... I'm optimistic.
As far as the sequel - it's very sad we can't see another Joker. As good as, for example, Johnny Depp is, he could not follow up this portrayal. They would be foolish to incorporate him.
I think the best option would be The Riddler, as characters like Mr. Freeze or Poison Ivy don't really fit into this realistic portrayal. The Riddler seems a natural choice. I hope it isn't catwoman I can't see her working in these versions... nor the penguin, really, not unless they completely changed his character. I remember Rad Al Ghul had a daughter in the comic books, that might work. And I wouldn't put it past anyone to bring Two-face back.
Side note - I kind of wish they hadn't made scare-crow into sort of a "b level" villain in these films. He has the capability to be an awesome villain.
Bane is a distinct possibility, but I never liked his character myself. I hope it is The Riddler (thought - per the rumor post, I don't think the accountant could be recast... these films draw far too big name stars for their main roles, and I have no idea who played the accountant).
WOW... long post... I really loved this film.