I'll be honest, politics aside, I'm a huge Johnny Depp fan and to me he has mastered every role he has done. I agree with your assessment of Ledger. Even in the movies he has done that I like he hasn't really impressed me. As long as Sienna Miller is getting lots of screen time in Casanova I don't think I will complain too much about who the main character is played by.
They'll screw it up. Casanova was essentially a very charming sociopath. They'll make him into a lover with a heart of gold. That's one I'll pass on.