I just read a real good Dark Knight review, which does a good job of explaining the complex topic(s) the movie attempts as well as the superb The Joker, in better words than I could manage.

Perhaps equally good is a hater review from the same publication, and happens to be more quotable:

This is a pulp experience that goes on and on and on, with the tired assumption that length connotes depth, and somber tones +++ and the occasional cackle +++ inject relevance and insight into an adolescent's approach to morality and ethics.

That quotable review by the way got comments which really deserves checking out, and among my favorites:

The thing that ruined the movie for me was all that impenetrable jingoistic bullshit about who was a hero and who needs a hero and what kind of hero, and who gives a shit? That and the bullshit about the noble people not blowing each other up. Both boats would have blown up at once with everybody diving for the trigger, yelling, "Me first! Fuck you!" Just like the Joker thought. What a lousy move for a great character to get stuck in.