[potential spoilers in this post, read at your own peril]
Great, great movie. Pretty morally ambiguous for a superhero film, which makes it even more fascinating to watch. A buddy of mine noted how much of a post-9/11 movie this is, as it invokes some of our privacy vs. security debates and fear of terrorists blowing up buildings and whatnot. Maybe someone's already spilled barrels of ink discussing the Joker is the ultimate terrorist, playing upon all our caricatures of them. Rene makes some good observations, especially about the more technical aspects of the film.
Rene is also right about the Joker and the screenplay writer's choice not to flesh out his back story. To take it a step further, in point of fact Nolan's Joker actively mocks the audience's expectation to be provided some sort of pop psychological justification for the Joker's madness. The Joker is not some mere lunatic with an agenda like Scarecrow (or, frankly, like Tim Burton's Joker); he's a primal force, a manifestation of Chaos, the perfect counter to Batman, the embodiment of cold, rigid Order. The two of them are locked in a perpetual struggle for the soul of Gotham, both utterly unyielding in their demand that the city conform to their ethos, while everyone else is stuck being pulled in either direction, trying to navigate a survivable path between the two.
This is what happens when an unstoppable force meets an immovable object.
Batman and the Joker share one big thing in common: both are fundamentally outsiders to the rest of humanity. The Joker lives off the grid, and has no records or Social Security number or last known address or even legal name. Batman is similarly separate from the system, even though he, at least, has an alter ego. Both are scorned by the city they are trying to change, labeled as "freaks." And, of course, both act in fundamentally deviant manners. After all, Bruce Wayne is a man whose parents were brutally murdered in front of him and who satiates his unquenchable trauma-induced rage by moonlighting as a vigilante dressed up as his childhood phobia. He is not exactly what we would consider a pinnacle of clear-eyed sanity, and no one highlights that fact as effectively as the Joker. There is a question in this movie as to which one of them is more self aware, as there should be.
This movie focuses on that struggle between these two primal forces via the battle for the soul of Harvey Dent, who is the real focus of the movie. What's interesting about it is, as those of you have seen the movie or are acquainted with the Batman universe know [spoiler alert!], the Joker wins that battle. In fact, in an ironic turn, it is the Joker's victory that prevents Batman from retiring.
For me, the most surprising casualty of The Dark Knight was actually my appreciation for Batman (1989). I'd never really realized before just how badly Tim Burton shortchanged the yin and yang rivalry between Batman and the Joker by allowing Batman a complete and total victory and carelessly tossing the Joker aside at the end of the movie. Also, seeing Heath Ledger completely disappear behind a thin, patchy coat of makeup makes the 1989 version look a little too much like Jack Nicholson playing the Joker playing Jack Nicholson. And yes, Ledger deserves an Oscar nomination for creating the best villain in any superhero movie ever.
Oh, and those of you who've been to Chicago will recognize Millenium Station, City Hall, and I believe W. Wacker Dr.