I’m going to be the jerk who doesn’t jump on the Heath Ledger bandwagon.
There’s a simple reason: for a movie touted as the one that gets contemporary comics’ depth, the Joker is a flat character. He is a nihilist. He’s a terrorist without an ideology.
Story-wise, it’s a decent conceit. How does Batman defeat, or at least outsmart, an enemy for whom chaos is an end in itself? But for an character, this nihilism flattened the Joker.
It even made his repeated attempts to tell the story of abuse at his father’s hand a throwaway, as if there was a conversation during the writing of the script where someone said, “The audience is going to want to know why the Joker is the way he is, why he likes knives, etcetera. We need to add some backstory, maybe just a line to head off that critique.” [Edit: My friend Jason, far more attentive to detail and better versed in comics than I, points out that the Joker provides multiple biographies, none of which would be considered true. So by definition it can't be a throwaway. My apologies.]
The late Ledger is getting rave reviews for his performance, but it doesn’t nearly approach the menace of a character with actual motives, with weaknesses to exploit, with obsessions beyond blowing things up. The Joker is a whack job. A petit Goebbels. He’s an entertaining villain, yes, and he’s fun to watch as the bad guy, but he’s too pure a villain. This isn’t what we’ve come to expect anymore in this genre—comics characters were invented when people still believed in unadulterated evil, but we’re past that, we live in a time when the phrase “the axis of evil” sounds pedantic.
The simple way to put it: the Joker is no Hannibal Lecter. He’s not a character as much as a plot device.