There were two things that kept me from giving Dark Knight a fifth star: First was the length of the film. I love long movies but this one seemed a tad too long. I guess it was the repeated scenes of mayhem and clown masks and disposable cops. Secondly and more importantly, the continuity of the wonderfully dark theme was often interrupted by a more traditional hollywood comic book take. The non-Joker villains, for example, were uncomfortably shallow and corny in comparison to the feel of the rest of the film. This film suffered from scriptzophrenia.
With that said, the movie was very entertaining. The visuals were terrific. The music was engaging. Love that Joker theme! The acting was top notch, especially Ledger (of course) and Aaron Eckhart really surprised me with his performance. Very well done. All in all, a very good film.