2. Mythos
Batman has a certain mythos surrounding him - his aura is a part of what makes him so frightening to criminals. No one knows who he really is, only what his motives are, which is to protect the innocent and bring all lawbreakers to justice. Lawbreakers cringe at the thought of him and have second thoughts of whether what they're doing is going to be worth it. Batman's mythos is going to take a serious hit if he is in the Justice League. Why? Again, look at the supporting cast. Do you really think in a world where Superman, Wonder Woman, and the Green Lantern exists, that a vigilante in a costume is going to warrant a second glance? If heroes flying in the sky, lifting cars, and putting out fires with their breath is an average Tuesday, then Batman is really the least of your worries. Put up Batman's mythos against the other members of the JL and see if it really holds its own weight. I want to be optimistic and say the Justice League movie is going to be OK in balancing out the team chemistry along with maintaining each heroes' status, but I doubt it. The Avengers made due with the Black Widow and Hawkeye because they used Hawkeye as a villain for most of the movie and used Widow as the Hulk's babysitter, plus Hawkeye and the Black Widow aren't major players in the Avengers, they're supporting characters. As a major character and founder of the Justice League, Batman's rep would take a huge hit and maybe never recover, and the reason why brings us to our last point...