Great review below, no real spoilers as such...
Here's the thing you must understand: even if you've dug up every possible trailer and clip of Heath Ledger as the greatest screen villain in any superhero movie ever, you really don't have any idea how good his performance is in The Dark Knight. The true strength of his Joker isn't his gallows humor one-liners or smart-a*s quips he delivers as he tears apart what is left of the fabric of Gotham City (looking more like its filming location of Chicago this time out than the juiced-up version in Batman Begins). The true strength of the final complete performance of Ledger's life lies in his much longer monologues. You see, you may think he's playing The Joker, but he's actually playing the Devil. He weaves truth and lies; every scheme is designed to wreak havoc on multiple layers, including ones that aren't always evident at first; he can also make you feel downright sorry for the guy as he weaves one of many autobiographies spins during the course of the film. He's a master manipulator, and wherever he is, he's the smartest guy in the room. Yes, he's insane, but he's going to let you think he's think that crazy equals reckless and unintelligent. He is neither.
Of course, a lot of what I've just discussed is as much a testament to the writers of The Dark Knight as Ledger. What Ledger adds to the mix is something he's clearly picked up from this version of The Joker. Just as Batman's archenemy has no moral code or fiber, Ledger has completely ripped to shreds everything he and we have come to accept and settle for when it comes to a portrait of evil. He has dismantled the status quo of how bad guys have been played in the past, and delivered a big whopping "F**k You" to every overacting, mustache-twirling ham who thinks that simply slicking back your hair and wearing dark clothes is the way villainy should be played in film. Watching Ledger move like a rabid animal or subtly flick his tongue like an angry serpent is to behold something you have never seen on screen before and probably will not again in your lifetime. Clearly, I don't need to sell you on how good The Dark Knight is. By now, you've probably read dozens of such reviews. What's important to distinguish is that director and co-writer Christopher Nolan's epic telling of the Batman vs. Joker saga is more than just the greatest superhero movie ever made (and that it certainly is), but it's the year's finest crime drama, greatest character study, and greatest acting performance.
Not to oversell Ledger's work (too late!), but seeing his version of The Joker actually made me mad at Jack Nicholson for not taking it far enough. Nicholson made a career out of being edgy and no-holds barred, but he and Tim Burton decided to make The Joker a clown instead of a true maniac. I'm sure it was not Nolan's intention, but his film made me dislike a film I once enjoyed because I now see that Nicholson's Joker is a pussy. I still think Michael Keaton was a decent Batman, but Christian Bale has such great pent-up (and sometimes not so pent-up) rage in him this time that no one can hold a candle to the dimensions he's adding to either side of his identity. Bruce Wayne gets as much time on screen as his costumed alter ego (maybe even more). And I never felt short changed, probably because when Wayne is in the foreground, it usually means that Michael Caine or Morgan Freeman or Gary Oldman is on screen with him. Each of these characters (respectively, Wayne's Butler Alfred, who gets an interesting sliver of a backstory this time out; the new CEO of Wayne's corporation Lucius Fox; and Lt. Gordon--soon to be Commissioner--who is almost Batman's right-hand man in the above-ground world) gets more screen time and far more to do. They are not simply background characters kept around for the vibe; they are vital pieces to the goings on here.
Maggie Gyllenhaal steps into Katie Holmes' role from the first film as Rachel Dawes, Wayne's childhood sweetheart and one of the few civilians who knows about his secret life. I guess I understand why the Batman films need a strong female presence, but the truth is, I've never liked any of the women in any of the Batman movies. Gyllenhaal's Rachel comes the closest to having a purpose other than slinking things up a bit and providing evidence that our hero is more than a shallow playboy (that said, Dark Knight's running joke about Bruce hooking up with the Russian Ballet is priceless). Rachel is torn between her true love and her new love, the "White Knight" district attorney Harvey Dent (Aaron Eckhart, in easily the finest performance he has ever given). The film's recurring theme of man's duality obviously comes to a head when Dent's face is severely burned thanks to a booby trap set by The Joker, but it's also driven home with The Joker himself, who makes the interesting observation that he and Batman are just different kinds of crazy. And Batman doesn't argue.
While Ledger's performance is the highlight of the film, the production's not-so secret weapon is its aforementioned screenplay (co-written by the director and brother Jonathan Nolan). There are some points in the writing that are absolute poetry. At other points, the humor is worthy of absolute fits of laughter. And the ending is staggering and unexpected, but the writers don't reserve all of the film's surprises for its final 15 minutes. What the brothers Nolan understand is that the Joker doesn't have to kill thousands of people to scare the s**t out of Gotham, he only needs to kill certain people, the right people, to bring the city to its knees. Unlike with Batman Begins, in which the evil scheme involved the slightly ridiculous idea of wiping out the entire city, I was far more angst-ridden by The Dark Knight's less complicated, but even more horrifying prospect of anarchy, chaos, mob rule, and citizen turning on citizen. The Joker is a far more effective terrorist than Scarecrow or Ra's Al Ghul. But it's just as fun to watch how The Joker fights face to face--he hides behind his cronies, pushing them between himself and whoever is about to pummel him. He rarely goes after an opponent unless their back is turned or their half out cold on the ground. And he never misses an opportunity to talk his way out of physical violence, which is not to say he doesn't relish a little knife play.
People keep asking me if The Dark Knight sets up an easy sequel. Of course it does. Most of the key actors signed to do three films. But much like the ending of Hellboy 2, the place Dark Knight leaves us indicates a third Batman movie will go in even darker and uglier places than what has come before it. You may have noticed I haven't mentioned the film's action sequences once. They are glorious, to be sure. But this film is that rare action film that won't leave you squirming and bored between the explosions and car chases. I told you this was a crime drama, and the emphasis is on the drama. The conversations in this movie are just as compelling as the fighting and killing and crashing and blowing s**t up.
In the end, I don't know if it's good or bad that we'll never get to see The Joker and Batman go head to head again in the current franchise. Of course it's terrible that Ledger is gone, but in a sentiment I'm sure The Joker could appreciate, he has left us wanting more. Even is Ledger were still alive, I'm not sure I'd want him to reprise this character. That would almost be too much of a good thing. What he gave us is more than we deserve or could have anticipated, and for that I'm overwhelmingly grateful. I'll say it one more time, and then I won't have to ever say it again about this film or Ledger's work: you have no idea what's in store for you when you sit down to watch The Dark Knight.