I still dont quite believe this movie is actually going to happen, there maybe alot of "will" for this movie to happen from fans and DC/WB looking at the much greener grass on Marvel / Disneys lawn, but despite reports, I just dont see it happening. Having a film in development is obviously not the same as filming.
Meanwhile if Man of Steel flops, that would kill this, they cant launch a Justice League movie on the back of a Superman flop, and the studio will need to wait a couple of years, before they can revive Superman.
As bad as the Green Lantern movie was, that is not a problem for this movie.
Assuming Superman is a hit, I think the biggest problem is making a new Batman work after the highly successful trilogy.