I think what people are most afraid of about Ben is that he won't lose himself in the role. Even in a suit and cowl nobody will see The Batman but instead Ben Affleck wearing the suit.
If he changes his voice I guarantee it won't matter, because any actor becomes largely unrecognizable once they put on the Batsuit anyway.
I'm with Damon on this one - people are severely overestimating the amount of acting skill it takes to play Batman [insert sarcastic Daniel Day-Lewis casting suggestion here].
Bruce Wayne is the bigger challenge, but I'm not sure how much an actor can actually "disappear" into that role. As talented an actor as Heath Ledger was, it's a lot easier to disappear into a role when you're in costume covered with makeup and able to affect a really weird voice - none of which applies to Bruce Wayne.
As far as I'm concerned, Affleck is a dead ringer for 70s Neal Adams Batman - so he's got the look covered - and he's at the very least a decent actor who has pulled off similar kinds of roles in the past (tough guy in
The Town, well-off social climber in
The Company Men and yes, vigilante hero in
Daredevil. Whatever problems that movie had, Affleck wasn't one of them).
So yeah, I'm not worried. Can't wait until they release a photo of Affleck in the damn Batsuit so we can finally have a little more to go on in this neverending debate.