As I'm sure everyone else has said, Heath Ledger as Joker is brilliant. Not the casting call, but Ledger's acting job, it's Fricken incredible.
I found it really interesting that Nolan intentionally neglected to provide an back story for The Joker. I'm sure it was the exact same case for the criminals of Gotham when Batman appeared. One day, he just showed up and completely ruined everything for them. One day, this day, Joker just showed up and changed the equation again.
He's not just some super villain for Batman to fight in another Batman movie.. he's a true nemesis. He exists because Batman exists. There's no other reason to Joker's plot or ambition. It's just to put wrong with what is right. To play around with society and knock it off it's tracks.
Everything else in the movie was muted compared to Joker. Everyone else took backstage and I really didn't care about them because Joker cast such dark shadow of their acting abilities. Why the hell does Bale talk like he's coughing as Batman anyways? Dent completely changed character so fast after becoming Two-Face.. yeah yeah, he's two-faced, but he went to killing cops kinda fast. Gary Oldman was fantastic as Gordon, but his character whispers all the time and doesn't stand out.
It's a truly sad affair that Heath Ledger killed himself earlier in the year with a drug overdose. All that potential and brilliance is completely gone now, but I guess it doesn't matter. It puts his performance completely stand-alone in the series. Between both movies, this feels like the climax of the entire new Batman movie series. What other super-villian could come along and mean more to the series as Joker? Mr. Freeze? Poison Ivy?
I think Nolan has outdone himself, and even though they are working on the next Batman movie already, how can it really compare? This first movie looks silly compared to this thanks to the Joker.
Joker, Joker, Joker.. that's all Dark Knight really is to me.