The first thing that comes to mind when I think of The Dark Knight a week after watching it is the great role played by Heath Ledger. The scene where he talks about his drunk father had me involuntarily touching my face, in anticipation of the gore that mercifully remains off-camera. All in all, a great villain. Pity we won't get to see him again.
Batman, in contrast, is not so great. He is overshadowed in so many scenes; can't blame him really, when you have folks like Michael Caine and Morgan Freeman as co-stars (not to mention The Joker).
The movie may be topical for an American audience, in that it explores things like how too much focus on security could lead to fascism, what people are willing to sacrifice for their personal safety [*], and so on, but these things are of not much interest to somebody who is just looking for a good action movie.
One thing that strained the credibility a bit was the power wielded by the Joker, and how he gets access to all the resources that he commands. Would have been easier to believe if he were a traditional comic book villain with superpowers or a super-weapon.
Oh, I've mentioned this before, but what's with these superhero movies where people are not able to put two and two together and figure out that the caped crusader bears a more than passing resemblance (mask notwithstanding) to the well-known local billionaire?
[*] The denouement of the ferry scene, while inducing the intended swelling of hearts in the audience, seemed a trifle contrived. Maybe it's just the cynic in me.
P.S. The high-tech kidnap scene in Hong Kong would have looked more natural in a Bond movie or in an MI sequel, IMO.