This reveal has been met with mixed reviews (mostly negative from what I've seen), and I felt it needed some context. Ben Affleck has definitely made some poor film choices (I try to forget the Affleck/J-Lo days), but if anyone has seen his recent films, there is no denying that he has grown significantly as an actor. Since we are talking about Ben as a superhero, everyone seems to jump straight to 2003's Daredevil, and logically so, but what you have to realize is that just because the film was bad (don't even get me started on the playground scene) doesn't mean the actor was bad. Daredevil was written and directed by Mark Steven Johnson, the same guy who wrote Electra and wrote/directed Ghost Rider, so obviously Affleck wasn't the problem here.
I have to believe that Snyder knows what he's doing. The way he did the casting for Man of Steel was inspiring, from the relatively unknown Henry Cavill to bigger names like Russell Crowe, Kevin Costner, Laurence Fishburne and Diane Lane. He knows how to surround his lead with enough talent so he doesn't have to carry the whole movie on his own. Nolan did the same thing when he started his Batman saga, surrounding the lesser known Christian Bale with four of the most recognizable stars in Hollywood today - Michael Caine, Morgan Freeman, Gary Oldman and Liam Neeson.
Obviously, Snyder sees something in Ben that made him believe he could embody one of the most recognizable and beloved superheroes in history. At the age of 41, Affleck still has the look that Bruce Wayne requires and if you saw him in The Town you would know that he is absolutely capable of filling out the suit. Affleck has the age, build, and presence to portay a veteran Batman convincingly, and at 6' 3.5", he won't have a problem standing toe-to-toe with Henry Cavill's Superman.
One issue I've seen several people bring up is that he's not intimidating enough to be Batman. I don't personally think Christian Bale was all that intimidating...until he put on the suit. It's the suit that makes Batman intimidating, for the most part, and until we see Affleck in it we won't know. However, I'm not convinced that him "not being intimidating enough" is actually an issue. You put him side-by-side with the likes of Superman and Wonder Woman and no matter how big he is, he probably won't look that intimidating. So, how do you remedy that? Focus more on his intellect and skills, and I wholeheartedly believe that Ben Affleck can pull off that style of Batman, which I believe is the way they will go.
And depending in the route that Snyder, Goyer and Nolan decide to take the story, having a bigger name like Ben Affleck as Bruce Wayne could pose an interesting struggle for Cavill's Clark Kent, perhaps even becoming a rival for Lois Lane's affection. Affleck may not be everyone's top choice, but the reasoning behind it is definitely interesting to consider, and even though it's still too early to tell whether or not Ben can pull off the role, I, for one, am interested to find out.