Every single one of Nolan's 'Batman' films felt more like "Nolan's dark gritty action hero series" and he just slapped on a Batman label to attract a larger audience. The Joker didn't feel like The Joker at all - I'm not saying Ledger is a bad actor, he was amazing, but the sort of personality they wrote in for The Joker in The Dark Knight felt like... well... this:
"I'm a murderous maniac with no clown elements to me at all, I just laugh now and then and put on some make up. But wait, I'm saying deep philosophical things! Maybe I'm not insane, maybe I'm just being completely honest! WOOAAAH! BE IMPRESSED AUDIENCE, THIS IS SO DEEP." He didn't feel like The Joker or anything like The Joker, just a maniac who took thought about human behaviour for a few minutes.
Ledger hits the exact right spot between killer and clown perfectly, and hell, Nicholson was pretty good in his own right too.