I wasn't on board with Ledger's casting either but he did a great job. I did like him before that but I wasn't some huge fan and I'm not a huge fan now. It's sad for his family that he died so young and I was shocked but I will never put him on a pedestal that I never had him on in the first place. I do find that to be a deeply dishonest thing that runs through the Batman forums.
He was great as The Joker and he was a talented actor but he isn't and never will be in my top ten favorite actors and he doesn't get into that list just because he killed himself...accidentally.
I would have said I was in the same camp. I never saw Brokeback mountain or anything else Ledger was noted for, in fact I'd only ever seen him in his earlier less serious films (
10 Things I Hate About You, A Knight's Tale) so I wasn't really a big fan or considered him an amazing actor. I'd heard good things, but that was it. When he was cast, I was quite indifferent to it.
Then I saw TDK, and was blown away. Every acclaimed actor has a breakout role of sorts - something that transcends the previous works they were in and marks them out as something special - and in my opinion, for Ledger that was TDK. The way he changed his voice, his mannerisms, his posture, his fidgeting, his look, everything - it blew me away. To me he was totally unrecognisable from the young guy in
10 Things...............
And not only that, but he managed to create a new and very sinister take on a character which had already been played (somewhat iconically, I may add) by Jack Nicholson, a performance which many people felt was pretty pitch perfect and couldn't be bettered. I've read many articles about the work and effort Ledger put into the creation of his take on the Joker, and his dedication was impressive. I particularly loved the scenes where he maintains the hysterical laughing side of the Joker perfectly, but a second later switches to this dangerous, growling personality - such as the handheld cam video where he has tied up one of the civilians posing as Batman. That to me exuded much more menace and danger than Nicholson's did.
It's just a pity his body of work was cut short by his untimely death. Many of the actors considered Hollywood greats (Pacino, De Niro, etc) have been phoning in their roles for years now, and I'd dearly loved to have seen what someone like Ledger - with a bit of vigour and youthful enthusiasm - would have done with his future roles.