The Dark Knight apparently being the most popular movie ever, reviewing it is a bit of an odd task. On the one hand, the receipts say everyone loved this movie. On the other hand, I didn't. (Warning: spoilers ahead.)
It's not to say that the movie was bad - it wasn't - but that it suffered from a bit of Schumacher syndrome (using too many characters) and a bit of something else - the tendency to confuse thematic elements until they become a mishmash of contradictory lessons. As Atrios and Alterman put it, it's a movie that endorses a libertarian fascism.
The central relationships in the movie are between the golden boy DA Harvey Dent and Batman and between the homicidal psychopath Joker and Batman. The former is a relationship built on who's the better leader for the lost populace of Gotham, the second built on the relationship between order and a chaos systematically built on destroying that order. In many ways, the movie feels like an academic argument rather than a fight for the soul of a city - leaders loftily arguing principles in life or death fights that spill over onto the populace rather than involve them; you realize ultimately that these people are all concerned with the functioning of a system as a thing in and of itself, largely unbothered with the people their actions affect under the presumption that they are, of course, doing the right thing.
One of the first conversations in the movie comes between Dent and Batman's alter ego, Bruce Wayne, as they discuss the Roman practice of selecting a single man under times of crisis to lead them. Rachel Dawes, the love interest of both characters, points out that this resulted in the rise of the Caesar and the fall of the Empire. The conclusion, oddly, is that when such men rise, their only options are to either die in service to the public or live and become tyrants of the selfsame public foolish enough to promote them. Batman and Dent, as competing Caesars, are the dictators of Gotham's fortune. It makes you wonder what about this city, lost without an authoritarian figure to lead it, is actually worth saving.
Along comes the Joker, a ghoul spreading chaos through the streets...and the most logical character in the movie. What The Dark Knight does with the Joker is what should always been done with the Joker: make him a character who believes that the true joke is order, laws, the artificial restraints of society, and that his purpose is to expose the joke, as violently and as spectacularly as possible. Batman's savage dedication to the preservation of order is what makes them such great enemies - they differ in method and purpose, but not so much outlook. But where the movie goes off the rails is in the resolution of their conflict; whereas the Dent and Batman relationship ends with the preservation of Dent's greatness in death, the Joker's ultimate fall comes in the choice of free people not to bow to savagery (long story short, the Joker sets up two boats with explosives; each boat holds the detonator for the other - either one of the two boats can survive, or both will be blown up at midnight, the joke being that the only detonators are the ones each boat holds and there is no external threat of detonation). Whereas our heroes debate over which one of them can save Gotham, the villain of the movie gives the people the chance to save themselves.
As you watch the movie (and in particular the ending, which declares that the preservation of our heroic myths is more important than the realization of truth), you start to realize why Frank Miller's The Dark Knight Returns was such a good realization of the character - the marriage of Miller's authoritarian worship to a character that is poisoned from his roots with the water of the authoritarian dream. A rich supergenius wronged by society's failures and armed with the resources to fix them, all outside of the selfsame strictures he seeks to uphold. It's the Bush presidency, minus the supergenius part.
It's a useful examination of the superhero as a construct and of Batman in particular. What makes the Joker a villain, Batman an antihero and Dent a supposed hero isn't how they view the city of Gotham. They all look on it with a similar disappointment and disdain for what it is and its lack of understanding of what it could be. What makes them what they are is how they seek to transform Gotham; the Joker by burning it to the ground, Batman through preservation by almost any means necessary and Dent by working within the system to reform and rebuild it.
If this is the salvation of a society on its last legs, it's hard to argue that the empire should persevere.