I like this trend towards super dark and realistic super hero movies. They're gritty and usually a powerful introspection into the issues in our society. But this isn't our society. DC Comics doesn't take place in our world. Gotham, Metropolis, Smallville, etc. are fictitious places that are reflections of our world. It allows authors to take liberties with the cities, like they did with Gotham in this movie. The liberties, though, make the movie too far apart from our society. It's TOO dark. TOO unhappy. TOO chaotic. It makes it too hard to relate.
Joker is the origin story of The Clown Prince of Crime, The Joker, Batman's most iconic villain. Obviously, it's going to be an image of mental illness, since Joker is obviously criminally insane. Knowing how violent and chaotic the character is makes watching the movie much more tense, since you don't know when he'll snap or how far he'll go. But the story is more interested in the environment that pushed Joker over the edge than the actual mind itself. His world is without love, without hope, without any light at all. Everyone is awful and, in reality, completely evil. No wonder he wants to see the world burn.
This departure from our world makes it very hard to understand why society hasn't already collapsed. Not even Batman can stop this world from tearing itself apart. I'm pretty sure that I'd be a part of the rebellion if I lived in that world, too. Some might say that we're heading towards this as a reality, but we definitely don't live here now. This is more of the RoboCop world of complete corruption and decay than the Gothic darkness of crime and deceit that is Gotham City. Since the environment has such an important role in the movie, I can't say that it's really a good movie. I'm glad I saw it, but I wouldn't see it again. I wanted a bit more of Joker's departure from reality, not the setting's departure from reality. If slow, demented, and dark is your cup of tea, you will appreciate this movie, but don't expect to love it.