There's not cover up for spoilers I can really see so if you haven't seen the movie please don't read this post!
Not that I completely disagree but the film wasn't as good as I expected it to be. There wasn't really any story of how the Joker came to exist. I mean, sure he was evil and psychopathic which is how a villain should be, but I didn't like the way the movie tried to focus more on his law breakings and town slaughter and the fact he just hated everything.
Ok, in the story properly, he was originally the dude who killed Bruce's parants and ran away, and then later on dropped into a tub of acid which is how the makeup mess up came to exist. I didn't see any of that, it was as if they coudn't be arsed with it. Twoface had it correct, though there's no sign of a sequel with him as one of the mains.
I'm not going to sit here and type like a marval fanboy though so I'll just go and spell check my typings.