It's in the Internet's nature to rage.
In short, people are pissed.
For a little perspective on the matter, let's jump in the not-so-way-back machine away from the Superman/Batman film to a little movie called The Dark Knight. Batman's iconic villain The Joker was the major casting decision for that film, and with beloved performances by everyone from Cesar Romero, Jack Nicholson and even Mark Hamill as the voice of animated Clown Prince of Crime, fan expectations were high. When Christopher Nolan and Warner Bros (the same people involved in casting Affleck, mind you) announced that Heath Ledger was cast in the role, everyone on the Internet looked into their crystal ball and saw a brilliant, Oscar-winning performance that would change how people viewed this seminal villain, right?
So, maybe the Internet give Affleck a chance to prove himself. Hey, at least his suit won't have nipples.