Let's get one thing straight: Ben Affleck is fine. I liked Argo, I liked The Town, and I really liked Gone Baby Gone. He's got a number of underrated performances, like Changing Lanes and Shakespeare in Love. And, yeah, he's got a couple Oscars.
But he's not Batman.
Will he be able to bring the appropriate gravitas to the story of a billionaire who dresses up like a bat and pretends to have a gravelly-voice and uses magic green rocks to punch an alien in the face? I don't know that he does. It requires a seasoned actor that is universally-respected, like Christian Bale (best known for being the bad guy in the Shaft remake).
Let's look at what is arguably the best Batman film, The Dark Knight. Obviously, the most compelling and interesting character in that film is Batman. His arc could only be played by the highest-caliber actor: At the beginning of the film, Batman has not punched anyone or yelled at anyone in a stupid voice, but by the end, he has punched a bunch of people and yelled all kinds of things with a stupid voice. Sure, all the heady and emotional arcs are given to the Joker and Harvey Dent, but Batman ends the film by thinking a girl liked him when she actually liked someone else. Also, he deduced the Joker's location by using magic cell phone GPS mapping. He's not called 'The World's Greatest Detective' for nothing, folks.
Oh! And he takes the fall for the actions of Harvey Dent/Two-Face so that Dent can remain a symbol of hope for Gotham, since there's literally no other character in the film who could possibly be responsible for so many mob deaths. Well, I guess the Joker might have worked, since he had already killed a bunch of mob people, and a whole bunch of other people on top of that, and absolutely no one would believe him if he claimed he was innocent of those crimes and that Harvey Dent actually did them all. But then we wouldn't have gotten that 'hero we deserve'/'hero we need' speech at the end, which was pretty cool.
Remember: Ben Affleck was in Gigli.
The actor playing Batman is always the most important thing in deciding how good that film will be. I mean, who do we blame Batman & Robin on? Obviously it's only George Clooney's fault. Did Joel Schumacher have anything to do with the movie being awful? No, it is only George Clooney's fault. That's why our concern should be about Ben Affleck and not Zack Snyder or the script by David Goyer (writer of Ghost Rider: Spirit of Vengeance and co-creator of FlashForward). Ben Affleck is the only factor that matters here.
What it all comes down to though is the fact that we need to get unreasonably upset about this before a single frame of has been filmed. We need to not give them a chance at all, since actors (like Ben Affleck) have never subverted the internet's expectations. Remember how upset the internet was when freakin' Heath Ledger was cast as the Joker? And we were right to be so upset (barring that Oscar he won for his universally-praised portrayal of the Clown Prince of Crime).
So let's do it again, internet. Let's show them how right we always are when we get mad about superhero movies.