I think THE DARK KNIGHT is as close as you could come to a perfect film, or at least a perfect superhero film.
In order to understand why, you have to look back at what Tim Burton did wrong with BATMAN RETURNS.
Firstly, he used the Joker for the first film. There wasn't much of a way around that, since it was the first film in the Batman franchise, and you really have to go all-in with the frst film to make it viable. So they had to use the Joker as the series' most-iconic character, and when it came time to do RETURNS, they didn't have much to go with. Nolan got away with using Ra's al Ghul and the Scarecrow in BATMAN BEGINS, because by 2005, Batman films were already ingrained into popular culture.
But more importantly, there is no story in RETURNS. The Penguin plans to frame Batman for a crime that he did not commit, but this is a serious problem because the ending is already decided: no matter what happens, Batman will find the evidence he needs to exonerate himself. I first came to that conclusion when I was six and saw the trailer to the film.
THE DARK KNIGHT, on the other hand, created a story where there was the possibility that Batman could not continue - and as THE DARK KNIGHT RISES establishes, Batman was not seen for eight years after the death of Harvey Dent. Christopher Nolan understood that you don't have the villain frame the hero. You have the villain take away everything he loves, everything he fights for, everything he represents, lower him down to the villain's level and see what he does next, which is precisely what the Joker does. Batman was created as a symbol to protect Gotham. The Joker understood this, and decided to prove that there was a man underneath it all, and that that man was like any other - filled with his own prejudices and values and emotions. He killed Rachel Dawes, brought terror to Gotham, lured people into doing things that they never would have done otherwise in an attempt to prove that they weren't worth saving or couldn't be saved to begin with. And finally, he tried to make Batman kill him (the only sure way to stop him), thereby breaking Batman's one rule and lowering him to the Joker's level so that even if the Joker was dead and buried, Bruce Wayne could never be Batman again. That's why the Joker was laughing instead of screaming as he fell off the tower, and why he was so impressed with Batman when he reeled the Joker back up. And then he forced Batman into a position where he had to make the city believe he was a killer anyway.
Compare that to BATMAN RETURNS, where the Penguin simply wants to frame Batman. THE DARK KNIGHT was much more interesting, and much more complex in its characterisation - and that's always a good thing.
It's not as good as THE DARK KNIGHT, but it's no slouch.
Personally, I think it fits in very well with the rest of the Nolan trilogy. Most of the returning characters feel like they're eight years older than when we last met them, and act accordingly, and the newer characters fit in, even if there are a few too many and one in particular gets an exposition dump early on. Some of the plot elements are a little unnecessary, and only really serve as a vehicle for the themes - the film isn't particularly subtle in showing the audience what the stakes are, which is a shame because it does overshadow some of the more-interesting aspects of the plot. Story-wise, it might be a bit confusing to begin with, but I think Nolan was still in INCEPTION mode, laying multiple plot threads that all spring together about halfway through, like a tightly-wound trap. By the end of the film,you might have forgetten some of the finer points that are laid down early on, though. Unfortuantely, the film does gloss over a few elements that it could have explored a little better.
As for the performances, I think pretty much everyone is dead on. I'm not that familiar with every interpretation of every character in the comics, but I did feel that Tom Hardy and Anne Hathaway really got into the swing of things. Bane might not be as interesting as the Joker, but I loved the way Hardy made him very eloquent in his speech and utterly brutal in his fighting. History will probably remember him as being a little trapped in Heath Ledger's shadow, but if he had played Bane before Ledger was the Joker, hsitory would probably be a little more complimentary. As for Anne Hathaway, I think she was let down by the script dragging her character back to a few standardised tropes, and she does very well, but when it relaxes is when she really gets to stretch her legs. Her Selina Kyle is definately my kind of girl, so long as she doesn't take my watch.
I know Nolan takes a lot of flak for not being able to do humour very well, but I think humour would really ruin it. The actors play the humour best when the themes are relaxed, and they clearly have a lot of fun with those scenes. There is one returning character who gets the best line in the film, and his self-satisfied smirk at being able to deliver a joke is what makes the delivery so good. I won't spoil it, but you'll know it when you see and hear it. Personally, I think it would have been a mistake to make this an overly-funny film. It would have taken away from the themes, and probably would have been seen as imitating THE AVENGERS, even if they had similar production windows. It's not laugh-out-loud funny, but there are a few lines that make you crack a smile.
Overall, I think the film works quite well. it fits in with the rest of the trilogy, and is probably the natural extension of the story. There are a few flaws, but by the end, it's very well executed. It ties everything off in such a way that Nolan and Bale can leave the series, but the films can continue in the same universe and allow for a new director and a new style. The big question are whether or not the studios will be willing to keep going, and if so, who they will get to replace Nolan. If talk is to be believed, Nolan will still be involved in the production, so if the studios do decide to continue with the Nolanverse, he'll at least be able to guide the boat a little bit.