Wow, I'm not even up to Dent and Rachel yet, this one's going to run a bit long sorry. I don't want to go so far to call Nolan a misogynist, but he doesn't put too many women in his huge, male-ensemble, films does he? Rachel, the character we're supposed to believe is driving the actions of both Batman and Dent doesn't make it out of the second act alive. As I said in the Batman Begins article, Batman has a host of interesting female characters to draw upon, but Nolan only uses a damsel in distress, it just grates when you think about it. Harvey Dent is amazing in this film, failing and "falling" through no fault of his own, finally getting what he wanted (we find out in the next film) at the cost of his own life. That the narrative doesn't centre around Dent is baffling, everything about him is cleverly laid in; we know from the "die a hero" line that he is prepared to give everything to his cause, something Batman doesn't consider until the closing minutes of the film. The coin is a fun element to Dent's character with the reveal of the two-heads coming at just the right time before the subversion at the hands of "Harvey Two-Face" and all the fun to be had there. Two-Face is probably my favourite Batman villain and The Dark Knight is one of the best interpretations of the character, I was actually annoyed when he was killed at the end, even if it is a good narrative choice for the film.
I almost feel like I don't need to talk about the Joker here, as so many people have done such a better job of it than I will. Seriously, check out the YouTube video "Movies with Mikey: The Dark Knight", he does about the best take on the Joker that I've seen, even if I disagree with his contention about The Dark Knight's protagonist. Personally, I still prefer the BTAS or Batman (1989) interpretations of the character, as they're more in line with my experiences with the comics, cracking more jokes and having a sense of sinister fun is essential, in my view, to portraying the Joker. The Dark Knight remains a magnificent film, even if each of my subsequent viewings turns up more issues for me to quibble against; I think that's the nature of examining popular culture too closely, everyone has a different interpretation and most are at least somewhat valid. Is Batman an Objectivist icon or hero of the people? Is the Joker the true hero of the film, or an avatar of lawless societal decay? Is Harvey Dent a failed hero, or was he destined to always be a martyr? I can't make these decisions for you, but I can give my opinion and encourage you to seek out more writing and academia about this film.