The Joker
Can an actor I always thought was kind of a mediocre pretty boy who belonged in teen romance flicks tackle one of the most interesting characters ever created (assuming we're talking Frank Miller)? I have no idea. After watching Hayden Christensen (who I think is comparable) turn Anakin Skywalker into a one-dimensial sniveling shitheel, I'm not particularly optimistic.
As an aside I just saw No Country For Old Men, and the Coens created one of the creepiest villains ever put on film. The Joker, as he's supposed to be written, should make that guy look like Fred Rogers.