It is, as it should be, PG-13. As many have said before, there's plenty of room for violence, darkness and intensity with a PG-13 rating.
I dunno what the people who so desperately crave an R-rated Batman film are thinking... I want a Batman film I can appreciate as an adult, but one that isn't restricted to anyone under 17 either. 90% of all the Batman fans on this board wouldn't even be here if they hadn't had the chance as kids or young teenagers to see the earlier Batman films.
I don't care about the extreme, bone-crushing, blood-gushing NC-17 fantasy Batman film these people have in mind, Batman is a superhero that appeals to (almost) all ages. I saw B89 as a 7-year-old and it didn't mess me up for life, I want kids today to be able to see TDK and enjoy it too. PG-13, done well, is the best of both worlds.