I think the particular kind of badness in Dark Knight is shown most clearly in the ferry scene. No one on the ferries has any good reason to believe the Joker is a man of his word. But everyone is written as believing perfect nonsense so the movie can pose a fake dramatic choice, supposedly with existential implications, no less!
I agree. You know going into a superhero movie you're going to have to suspend disbelief - but this plot point (in fact, the entire last act) was ridiculous on too many levels.