Big question yet to be answered: Is Man of Steel the first installment in a Justice League series?
Second Question: Does anybody want it to be? To put it another way, do we want to see a Justice League led by this Superman?
Third question: Do we really care if they make a Justice League movie at all?
Oliver and Ken Mannion were on top of this a long time ago. On the whole, starting with Bruce Timm’s Batman: The Animated Series, the cartoons based on the DC Universe have been better than all the live-action movies, except for The Dark Knight, and some of the Marvel-based ones as well, with Batman: Mask of the Phantasm being the best of the best. They’re including the first two Christopher Reeve Superman movies and Batman Begins in their judgment. As far as they’re concerned, there’s just about no point in trying to make a Justice League movie because the cartoon series has been there, done that, and done it very, very well, far better than any movie rushed into the theaters just to compete with The Avengers is likely to be.
So they were glad to see this article by Tim Donnelly in the New York Post, Why They’ll Never Get a ‘Justice League’ Movie Right and they’re pretty much in agreement with the whole of it.
For the record, according to the Mannion men, Bruce Conroy is the best Batman and Mark Hamill the best Joker. Heath Ledger was great, but they feel his Joker was a completely different character.
If you'd like to see samples from the cartoons, Oliver Mannion has helpfully provided links to some video clips on YouTube:
From Batman: The Animated Series:
This one's Ken's suggestion. From Justice League Unlimited: