I thought the movie was good, but I saw it just after going through something very personal, so I was in a darker place than most people seeing the movie.
Here's my thoughts... and there's no spoiler:
1. I disliked Christian Bale as Batman's VOICE. His performance was great, but his voice really threw me off totally. I also didn't feel like his character was developed enough. I guess it was really well developed in the first movie, so there's no need to tread on it, but some background would've been nice.
2. The depth of the Joker's psychosis really wasn't explored enough. Sure, he's got no real motives other than destruction, which makes him the ultimate socio/psychopath. However, I really think that his character would've truly benefited from having a hopelessly devoted sidekick or girlfriend. Someone who would do absolutely anything for him, and loved him and was just 100% trapped in his spell. Someone who the Joker would ultimately sacrifice, showing how deeply fucked up he really is. Killing a bunch of people you don't know and don't care about is easy in theory. Regular people do it every day. But killing someone that shows nothing but forgiving, non-judgmental, and hopeless devotion to you is truly the act of someone extremely fucked up. Like, killing the family dog -- it takes someone of a different caliber to do something as messed up like that, but those people exist. With any socio/psychopath, you have people that follow them and are devoted to them. That's what allows them to have power. It's almost impossible to understand their magnetism, but showing it, explaining it, would've really given Heath Ledger's role a lot more substance.