I think one of the things that interests me about Gyllenhall in the part is that he can do ugly.
The mans a good looking dude, and can carry of the matinee idol routine, but his past performances show he can go to very ugly, nasty places in his roles, both physically and mentally. This, I think, is very important for a convincing live action performance of Batman.
The same was true for Bale.
Affleck has never struck me as an actor who can do ugly. This was always the doubt for me when he was cast. Hes a square-chinned Hollywood icon, but his body of work have never given me the impression he could embody Batmans darkness of character properly.
I think you need an actor who can play villains as well as heroes to play Batman.