I've been thinking about this, with all the success that the Marvel Studios has earned (and well deserved) with their franchises, I don't really think DC and Warner Bros. should rush things up for a Justice League film.
For thematic purposes, Nolan's Batman works better isolated from other superheros. I think DC characters lend better for solo films. I think what DC and Warner Bros. should do is to exploit the richness of their characters in separate ways. Let Superman have his own adventures on Metropolis, let Wonder Woman battle the perils of man's world. Let Flash show his wit while confronting the perils of Central City.
I though Green Lanterns film would be the key that would open the door to all of these possibilities, but failed. The film had potential that went nowhere. DC needs to take risks, and get talented people for their films, but not necessarily to build up a Justice League movie just because it need to get on par with Marvel.
Yes it is working great for Marvel, and I enjoy their films, but I think DC needs to do something different. I don't think a Justice League movie will be the answer.