I haven't seen The Dark Knight yet. I will see it soon. I have read a lot about it, and most people who saw it seem to agree that it is good. Their reasons, however, are interesting. Many say that The Dark Knight is dark, gritty, and brooding. It seems as if any film that is violent, dark, somber, serious, and cynical about human nature, is lauded by critics.
I have read somewhere that so-called erotica is just a name for pornography that is tweaked for upper class consumption. Erotica allows individuals to indulge in pornography without feeling guilty that what they are consumed is sinful.
The same might apply for violent movies. Many serious critics disapprove of the gratuitous violence and gore in teen slasher movies like House of Wax. However, many of these critics praise violent movies like Road to Perdition and Mystic River (both good movies, by the way). These dark, gritty, and brooding films popping up all of a sudden are like the erotica of violence.