Went to the midnight showing on Thursday night with some friends.
First of all, pretty much everyone is right in that it was a great movie. It was completely engrossing (and terrifying) throughout the entire two and a half hours, and I thoroughly enjoyed it. Despite significantly more action than Batman Begins, it managed to brilliantly develop a complex plot and several main characters. Despite the title, the film was actually just as much about Harvey Dent, The Joker, or James Gordon as it was about Batman/Bruce Wayne. Heath Ledger was truly amazing as The Joker, and the rest of the cast was solid as well (I mean really, can there possibly be a better Alfred than Michael Caine?).
Despite all the ingredients of not only a great superhero film, but a great film in general, there's one key thing that indicates whether a film was great or not: how memorable it is. It's been over a week since I've seen The Dark Knight, and I'm still thinking about it! That is why it's so good. When your thoughts constantly return to the twisted and depraved acts of the Joker, internal struggle of Bruce Wayne, or tragic fall of Harvey Dent, you know there was something about The Dark Knight that made it awesome.
*note* Reviewers are right in that this is not a kids' movie. Aside from a few dirty words, The Dark Knight is very violent and downright scary at times. I still get chills whenever someone licks their chops like The Joker. Anyways, there are plenty of places to get more info on content. Just use some wisdom and discernment before seeing it.