All right, I think we're past the point where we have to worry about spoilers, but if you haven't seen "The Dark Knight" yet, then you might want to hold off on reading this post, because I plan to talk with no concern for giving away plot points, large or small. On the other hand, I saw the movie a week ago, so my memory might be a touch fuzzy in places.
I would like to make the argument that, as much as I enjoyed the movie, that I came away with the conclusion that the Batman, as Batman, simply doesn't work on the big screen, although, strangely enough, the Joker does.
"Batman Begins" and "The Dark Knight" work largely because Christian Bale is a good enough actor to make you forget that he isn't really playing Batman. While Nolan and Bale deserve credit for sticking to point #2 of my "Batman Best Practices," Nearly all of the Batman movies filmed to date, whether Burton, Schumacher, or Nolan, seem to deal with Bruce Wayne's angst over putting on the mask, and his desire to create a situation in which he wouldn't have to be the Batman any more. In Batman as Batman, this conflict doesn't make any sense. It is strongly hinted that even if Bruce Wayne's parent's hadn't been killed, he would have been something like the Batman. Maybe not quite so dark or violent, but every bit as obsessive over the application of his own sense of right and wrong. (This idea is contained in both points #1 and #4. The death of Thomas and Martha Wayne isn't what made Bruce Wayne the Batman. He did that to himself.)
There are elements of the Batman that simply don't work in a live-action summer blockbuster. Not since Tim Burton's 1989 "Batman" has any film even much bothered to try to show Batman as a detective, and Nolan's Batman never really has to figure anything out. Ra's Al Ghul just shows up at his house, he discovers the Scarecrow's operation by accident after following Rachel Dawes to Arkham, and he's so helpless in dealing with the Joker that he has to create a universal surveillance system in order to find him. Nolan's Batman is more of a special-ops soldier. He's really good at designing small tactical operations, like when he pulls the crooked banker out of his building in Hong Kong, but he's not a detective. (But who can imagine a "detective" Batman movie? There are plenty of great detective television shows and films, but Batman as Sherlock Holmes, throwing hardly a single punch, probably wouldn't pack the theaters.)
Secondly, beyond the fact that point #1 of my best practices cuts off the "should I be Batman" conflict that nearly every film seems to feel the need to use, no actor would ever agree to use the comics' primary visual cue of the point: Wayne wears the Batman mask even when he's hanging out by himself in the Batcave. In real life, as an actor on a film set, that would be damn uncomfortable, and would eliminate most of the actor's face time. Whatever you want to say about Keaton, Kilmer, Clooney, and Bale, none of them are Lon Cheney, willing to disappear behind makeup for an entire film.
(Side point, or addendum to best practices, call it point 3.1—the bat-suit is not armor! In the modern comics, Wayne wears kevlar beneath the costume, but it should be closer to cloth than plastic. Light, slient, and not cumbersome. This, again, may simply not work on screen.)
In opposition to all this, it's interesting how faithful Nolan's Joker is to the comics, and the way that they are able to adapt key elements of the comics in realistic ways. In the modern cable/broadcast/satellite TV world, I don't know that it makes sense for the Joker to hijack every TV set in Gotham to announce his crimes, but Nolan makes an excellent choice in having the Joker videotape himself doing terrible things to people and sending it to the news outlets. the Joker doesn't have to hijack the broadcast when the networks are more than happy to air the footage of their own volition. It's a chilling commentary on our contemporary media culture.
Other key elements that Nolan is able to make use of are the Joker having a "multiple choice" past as shown by his infinitely adaptable "how I got these scars" story, and his aptitude for disguises. On the other hand, a Joker who is ultimately unwilling to kill the Batman doesn't make any sense. I'm willing to buy the "I don't want to kill you. You complete me" line as a lie containing a deeper truth from a character who is prone to such things, but the Joker's "you won't kill me, I won't kill you" when he's hanging upside down at the end doesn't make sense, and is the opposite of the long-delayed but ultimately inevitable dance of death that makes Alan Moore's "The Killing Joke" work. The Joker needs the Batman in order to exist AND he wants to kill the Batman AND he wants the Batman to kill him. That contradiction is at the root of his insanity, and it's what makes the character run.