Here's the deal.
I believe in Zack Snyder. The man is a comic book fan and as far as casting goes he's never had a miscast, at least not in a CBM. The man managed to get some huge actors together in Man of Steel - Kevin Costner, Diane Lane, Russel Crowe and Michael Shannon all of which I'd never thought I'd see in this type of movie. Looking back he's had some great casting hits - regardless of what you thought of "Watchmen" and "300" as movies, it's hard to deny that each cast member was perfectly suited for their part. So have a little faith in the man - he's probably interviewed at least a couple of actors and has a pretty good idea of what he's looking for in his version of Bruce Wayne/Batman. I admit I would've preferred to see Richard Armitage as The New 52's Batman but I'm not dismissing Affleck without at least hearing him read some lines first.
That said, here's a few ideas on how to overcome the main issues fans have with Affleck:
1. He doesn't have what it takes to play Bruce Wayne
While I partially agree that Affleck doesn't quite project the confidence, charm and inner darkness I'd expect to see in Bruce Wayne, you're forgetting that he's not shooting the movie in his pajamas. There's a whole make up crew and a costume crew that will be working on bringing out the best in him. Give him a stubble and a messy haircut and just enough make up to sharpen up his facial features and you've got a decent Bruce Wayne right there. As for his onscreen presence, a lot of fans seem to be forgetting that he'll probably be side by side not with Superman but Clark Kent - the clumsy, nerdy reporter from Smallville we only got a glimpse of at the end of "Man of Steel". It's not that hard for Bruce Wayne to stand out when faced with this classic version of Clark Kent.
2. He doesn't have what it takes to play Batman
Let's face it - it doesn't really take that much acting talent to be a decent Batman. Half your face and I mean the half that's actually most involved in facial expressions is covered and you don't have that many lines. It's obvious that Snyder's Batman will be very different from Nolan's version so I think it's safe to assume that Christian Bale's growling voice is off the table. But this doesn't mean that some modification won't still be in play. There's nothing to worry about in terms of physical transformation either - Affleck is 1.92m and he's got plenty of time to bulk up plus they'll probably be going for a sculpted body armour type of suit which would easily make up for anything he's missing in the muscles departament.
And let's not forget that Affleck really looks a lot like the classic square faced Bruce Wayne from Batman TAS and even the Arkham games so this might mean that we're finally getting a modern day interpretation of that same character. They might even throw in the white eye lenses.
And in conclusion - there's been a lot of hate towards Ben Affleck almost as if he's got the part in some unfair manner and plans to taint it with his Benaffleckness. While I haven't seen any of the movies he's directed and gotten Oscars for I know that he's made quite a name for himself in the movie industry - both as an actor and director. Before you villainize him as a thief who's snatched this part from more deserving actors think on what he's got to win and lose. Sure it's 60 millions worth of cash but does he really need that sort of money for a movie if it's going to blow him back to square one ruining every inch of reputation he's built in the last couple of years? I don't think he'd be willing to take that risk and gamble his career unless he was certain he could pull it off. And neither would Zack Snyder who's proven he knows what he's doing when casting someone for a superhero.