Great movie, it's a great ride (if not overly long), blew my mind in the theater, but these days it's hard for me to take the dialogue seriously. Ledger's Joker performance, and most of his lines, are pure fucking gold. Without him, I think the movie would not have been nearly as praised.
Every other character sounds totally unnatural. Or maybe it's that they treat the audience like idiots. The lines are bad. So much time is devoted to explaining what's going on, even thematically. Alfred explains the Joker, Gordon explains the Joker, Joker explains himself, Batman explains his dilemma, Joker explains Batman's dilemma, Harvey explains all of his actions, Gordon explains his ruse (which made no sense and was just there to cheaply get the audience to cheer), Gordon explains the fucking title of the movie--problem is, just because you explain something doesn't mean it makes sense, especially in Harvey's case.
I'd say it deserves high praise in genre, but so many would say it's because it isn't in the superhero genre that makes it so good. Okay.