See, I didn't like it when they cast Ledger as The Joker, but I wasn't too worried because, hell, Ledger had made some movies I DID enjoy to some extent.
In every movie Ben Affleck has ever stared, the few I enjoyed (right now only Dogma comes to mind), he wasn't really the character that gave the movie a SOUL. Even in Argo, I always criticize that the movie movie was praised for the wrong reasons, everybody loved it because of the events and the events were a merit of HISTORY, not Affleck's acting or anything movie-related (a good movie, don't get me wrong, but ordinary).
Anyway, I look at Affleck and I see the sort of schmuck that hits on your girlfriend in a party, then, after causing some trouble, moves to the next girl and, being such a pain, the bouncer has to kick him off the place. He's always wearing the same expression on his face that reads "I NEED to look serious, otherwise I'll look like a freshman and no one will take me seriously! See, here I'm serious and I'm ALWAYS worried about something". Oh, and he already played Batman, I mean, Marvel's version of him, as in "dark-on-the-edge-of-good-and-evil vigilante", Daredevil, so we all now how serious he can take a taciturn superhero role, yeah...
I'm not a big fan of Bayle either, but he had the "Wayne vibe", fuck, even CLOONEY had that vibe (the movie, sadly, lacked everything else to merit it as a movie per se).
Ps: I hope I'm VERY wrong, at least I'm "under hyped" meaning it'll be easier to enjoy the movie lol