The thing is, Nicholson was great at the time. But when I look back at it, I realize that it's basically Nicholson playing Nicholson and just happens to be dressed up as The Joker. Sure, he was funny and had some great one-liners, but Jack really got the psychotic feel behind the smile, as well.
Ledger's role, on the other hand, is not meant to be taken with a single smile. And really, that's how The Joker should be portrayed. He's this chaotic force of nature. I was excited when Ledger was cast, but I remember friends saying "But...Ledger's not a funny guy." My response? Good! The Joker in the comics (at least in the last 15 years or so) has been portrayed as a huge threat, the kind that even fellow supervillains trade horror stories about. That's the kind of Joker I was looking forward to with Ledger's role.
I think Burton's movies were great at the time, but time (and comparison to Nolan's) have not aged them well. My biggest beef with those series of movies was the portrayal of Commissioner Gordon, who looked like a bumbling oaf instead of the hardass detective we saw in Nolan's films.