I just always think it's a recipe for disaster to cast a very well-known actor as a superhero.
It just makes it really hard for me, and I'm sure a few others, to see the character and not the actor. Guys like Henry and Bale weren't TOO well known (at least not to me) when they showed up in costume, and as such it's easier to see Bruce Wayne as Batman and not Affleck as Batman. I kind of wish they had gone with someone unknown, but if it's a Batman VS Superman movie and not a Batman AND Superman movie, then I already have no expectations. I'll wait for it to come out, but to me, a "vs" in the title is a bigger red flag than an Affleck in the suit.