Let's just say that portraying a character in a comic book movie is not what can be considered a "difficult role". They are one-dimensional stereotypes, as they are required to be by the limits of the medium.
If you want an example of Ledger's good acting, watch Brothers Grimm. It's a silly movie, but Terry Gilliam gets 300% out of his actors.
But then again, most people on this forum judge movies solely on the level of exaggeration in its special effects. As indisputably evidenced by the "Movies Seen Recently" thread.