Main problem: expository verbosity
The characters do what the movie wants them to do. The movie wants to ponder certain morality conundrums, so its characters, mostly Dent and Joker, but also Batman, Gordon and Alfred, spurt out expository dialogue to move the plot forward and give the characters meaning and motivation.
Example:
Batman: This city just showed you that it's full of people ready to believe in good.
Joker: Until their spirit breaks completely. Until they get a good look at the real Harvey Dent, huh? And all the heroic things he's done. You didn't think I'd risk losing the battle for Gotham's soul in a fist-fight with you? No, you need an ace in the hole; mine's Harvey.
Batman: What did you do?
Joker: I took Gotham's White Knight and brought him down to our level. It wasn't hard. See, madness, as you know, is like gravity: all it takes is a little push. (Laughter)
***
etc. etc., and loads of lecturing, instead of allowing the viewers work out the morale of the story themselves.