I liked the movie a lot. I'd rate it at 4 stars, possibly higher. I was never bored during the movie, and it didn't feel too long to me. I thought the violence was appropriate for the Joker. There were too many false endings.
There were two particular things about the movie that I didn't like.
First, and of less importantance, I don't think there was enough development of Harvey Dent's psyche to make his actions at the end believable. For me to believe that, I think the viewer (though not Gotham's populace, obviously) has to be made aware that Dent has some serious psychological problems which in the end cause him to crack and become seriously insane. I think the coin-flipping in the first part of the movie is not enough, because the coin-toss is fixed. I don't see enough of his pysche to see how he goes from the completely non-random fixed coin to actually letting the now blackened coin randomly decide the fate of innocent children. Mobsters, yes; innocent children who could not possibly be to blame, no. At first I wasn't convinced that his going after Gordon and Batman and letting the Joker go was plausible, either, but I can understand how Dawes's death and their involvement in it could send him after Gordon and Batman, and if he's going to be consistent about the coin I can barely see him letting the Joker off. But after Gordon's family? No.
Second, and more importantly, I think having Batman and Gordon decide for Batman to take the blame for the death of those Dent killed (and Dent?) plays into the tendency of governments and officials to lie to its citizens "to keep them safe" and in particular to lie to them because the government doesn't believe they can deal with the horrible truth. I'm not convinced that Batman would really believe that the citizens of Gothom City would be so traumatized by Dent's breakdown and madness that the city would be seriously impaired, and that even if he did I'm not convinced that he would see it as the correct course. Lying to the public "to keep them safe" just makes the public powerless, and I think Batman isn't about making the public powerless, he's about protecting them when their normal protections aren't working, for whatever reason. He's not going to keep the public weak just because he likes being a vigilante, he's going to help make them strong.
I've read Batman comics and I own the Batman RPG (it was sort of an intro to the Mayfair DC Heroes game) though I've never played it, but I'm in no way a Batman scholar, so I may be misinterpreting things, or perhaps it's just that Batman has varied so much over the years that you can find support for any view, but this is what Batman looks like to me. I don't see Batman deciding to take the blame for those murders just to coddle the public.
Now, the whole idea of the perviously adoring public turning against Batman does lead the story into interesting places, but I think that this could easily happen anyway. For instance, if a a charismatic reporter or official carries on a campaign emphasising the negative aspects of Batman's vigilantism - can you say "collateral damage"? - I could easily see the crowd turning against Batman. (Although that might have too many echos of Peter Parker and J. Jonah Jamison.) But I think some other plausible method could have been found.