Let's get one thing straight. Christopher Nolan's The Dark Knight is not the new Batman film. The character who calls himself Batman in the film bears a passing resemblance to that superhero, it's true - he has the flying cape, the mask with the pointy ears and a certain facility in hand to hand combat. But there the resemblance ends. This Batman is nothing like the noir crimefighter we know and love. This Batman is all sentiment: he's lovestruck, he has qualms; he's so bad at concealing his secret identity that a quick audit of the Wayne Corporation's accounts will tell you who he is; he likes cooperating with the police and putting criminals in jail but dislikes killing, in fact, he gets so upset when he realizes that people are being killed because of him that he has an existential crisis and decides to give himself up. He doesn't quite say "with great power comes great responsibility" but you can see him thinking it, and it makes you realize that with a different costume this guy could be Spiderman or Captain America or any of a half dozen other superheroes. In short, he's a nauseatingly all-American goody two-shoes, and the most vanilla incarnation of the caped crusader since Adam West's 1966 appraisal of the role. Hell, even George Clooney was edgier than this guy (though only just).
No, what The Dark Knight really is is the new Joker film. Because Heath Ledger's Joker isn't just far and away the darkest villain ever to appear in a Batman film, he is the scariest, most bruisingly magnificent vision of pure evil to appear on screen since Hannibal Lecter. Sadistic, psychopathic and machiavellan, Ledger's Joker is chaos brought to life, a whirlwind of destruction that whirls through the film leaving human debris in its wake. To call him a megalomaniac would be to miss the point, so powerfully does he tower over his opponents that what you're seeing is not delusions of grandeur, but grandeur itself. Callous violence one expects from a comic-book super-villain and a certain amount of maniacal bloodthirstiness is par for the course, but it's the insidiousness of the Joker's villainy, the casual cruelty that hides behind the grotesque slurping exterior, the sense of murder as a vocation, the ease with which the Joker finds the weakness of his opponent, or uses his strength against him, that is both hypnotic and chilling. Ledger's Joker is a modern-day Iago - a villain of such elemental proportions that he transcends the trivial details of plot and action. It's an indelible performance.
Perhaps a little too indelible. The trouble with Ledger's Joker is that he makes everyone else in the film look tame by comparison, so that the movie seems deeply unbalanced, with the viewer wishing the good guys would go away so he / she could see more of the Joker. It is a good thing that the plot, for the most part, goes along with this, ceding almost all control to the Joker and reducing the good guys to little more than trained circus animals dutifully jumping through the hoops the villain lays for them, otherwise the contrast between Ledger's dominance and the Joker's subservience would tear the film apart. As it is, the Joker seems to preside over the action like some anarchic deity, some leering Beelzebub or red-nosed Baal. Even the end seems not so much a defeat as a conclusion - the Joker gets his comeuppance, of course, and Batman 'wins', but it is a pyrrhic victory, and one is left with the distinct impression that the Joker, having inflicted irreparable damage on Batman, has got exactly what he wants.
But all that is not enough. The central conceit of the film (aside from extremely overdone references to the 'two sides of a coin' analogy ) is that the confrontation between Batman and the Joker is a face-off between two people who play by very different sets of rules (the Dark Knight and the Joker, chess and poker, see?) with the obligatory connections to 9/11, the war on terror, etc. etc. In truth, however, what we see is a confrontation between two different genres - a scarily real villain matched against a cast of make-believe heroes. The genius of Jack Nicholson's portrayal of the Joker, two decades ago, was precisely that he was a comic-book villian; a bizarre, larger-than-life arch-fiend, who seemed to have stepped straight out of an animation. He was psychotic and scary, but in a way that conformed to the boundaries of the genre, so that the battle between him and Batman was a comic-book battle fought by comic-book rules. And every Batman villain since has more or less conformed to that standard. What Ledger does, however, is step out of that frame, and give us a villain who is not so much larger than life as viciously life-like, a villain who belongs in a hard-nosed crime thriller, not in the make-believe world of Gotham, a villain, in short, who makes the rest of the film's enterprise seem ridiculous by comparison. Watching him toy with Batman is like watching a man-eating tiger take on Mickey Mouse.
To be fair, Nolan tries desperately to find a way to match the gravity of this Joker. The body count gets ratcheted up, both qualitatively and quantitatively, there's a great deal of heartbreak, a lot of 'serious issues' get discussed. There's even a 'people are fundamentally good at heart' scene, which I suspect is meant to be a poignant affirmation of some sort, but comes across as just being corny. None of this does any good. So wholly does the spirit of the Joker rule this film that all Nolan's attempts to counter it just end up weighing the film down further - robbing it off momentum without adding anything of consequence. Nolan even goes to the trouble of giving us a second villain, but compared to the Joker what's-his-face seems so trivial that his scenes prove unintentionally hilarious, as though someone had wandered off the sets of the Mummy  to provide light comic relief.
All in all, The Dark Knight is a film worth watching only for Heath Ledger's performance. But that performance alone more than justifies the price of the ticket.
 People who've seen Sholay - which is probably all of you - may recognize a familiar plot device.
 Did you know they're making a third Mummy film? I saw a trailer for it today. So apparently Shrek Goes Fourth will not be the most unnecessary sequel ever made. Pity.