First, don't get me wrong, because I think RDJ's performance is a triumph and I'd like him to win, and I think he achieves much of what Ledger achieved.
What Ledger did that was great, and is most difficult and few actors achieve it, is lose himself. And I don't mean that in the typically pedestrian sense in which armchair critics say to mean you couldn't see or recognize the actor.
I mean Ledger wasn't there, almost literally. What most don't understand about great acting, even actors, is that to pull off a fantastic performance, especially in cinema when the camera is on your nose and the audience is in your face, is that your consciousness cannot be there.
Many actors will perform, or be a character, or even live as one. But in their heads, in their minds, in their consciousness, they are still them being that character. They are trying
. They are themselves doing something.
A truly great performance is when that actor is on camera, they are not themselves, they must be the character in presence and act, but also in thought...and never to be thinking to be thinking as the character who is doing. This is a very rare gift that is not often seen and rarely done. It would have been amazing to be Christopher Nolan behind that lens and seeing the magic between action and cut.
What makes Ledger's performance great is he's double acting. The Joker is a liar. He's always scheming. He's borderline insane, and everything he does he's doing his best to hold a veneer of sanity to be able to accomplish his goals. In every scene Ledger is in, he is the Joker in presence, but you can also see him as the Joker in mind...and that is a performance worthy of an Oscar.