[AChimp]

I remember everyone complaining when they announced Tobey Maguire was going to be Spider Man, and then it turned out that he was really good at playing the character.

Everyone is basing their opinions on what they've seen of Ledger act in so far. No one has seen the auditions he did for the Joker; Batman Begins was pretty damn awesome, so I'm sure the filmmakers know what they're doing for this.