Ben Affleck, as far as I know, was never officially tied to the project. He's mostly turning his sights towards directing these days anyway. I think he's going to do less acting and pursue that avenue. But what's wrong with Matt Damon? He's a good actor. As much as I love Captain Kirk, William Shatner is just an OK actor, not a good one.
As far as what they want to do, J.J. Abrams wants to revitalize the franchise. He's always wanted to do a ST movie or show and, now that he has his chance, he's going to do his best to make it good. Obviously, the powers-that-be at CBS/Paramount want to revitalize the franchise, otherwise they wouldn't have canned Rick Berman (Roddenberry's hand-picked successor) and hired J.J. Abrams, who brought his own production team aboard.
Either way, Trekkies will cry. They're crying already. Many of them, without even having seen what the movie is going to be about or even who is going to be in it (besides Leonard Nimoy and that dude from Heroes,) have already decided that it's going to suck. And the funny part is, they're mostly basing that off of what Star Trek became under Rick Berman and he has absolutely nothing to do with the project, so it's pointless to even compare the new movie to what was done before. But people always do that about new movies and shows.
