Bruce Wayne is part of the formula that makes the Batman franchise successful. Yes, he's had people fill in under the cape and cowl with mixed results, but he was always back in the mantle soon enough.
No Batman film would tick without a villain, and in the best personification of any comic villain Heath Ledger takes the prize (and most likely an Oscar). Not because he's dead -- because he's that good.
I love the way Michael Keaton plays Bruce Wayne. We usually see him as a socialite, a playboy, and I suppose you could say he is here, too, but it's a loose, complex take on Wayne. Still, the plot keeps fraying.