It's beginning to sound like Justice League of America is going to be more like Challenge of the SuperFriends than Justice League or Batman Begins. According to a source at Moviehole, the movie is going for a tone similar to that of The Rocketeer or Sky High.
Apparently the characters will all be introduced quickly - no origin stories here, each member of the Justice League is already established as a hero. We'll get a chance to see each hero doing their own thing individually. Green Lantern will get to battle a giant monster and fly through space, Aquaman battles a sea creature, the Flash gets to outsmart the Trickster and so on.
We quickly move on to be introduced to Max Lord, our villain. Max is watching what seem to be Superman, Batman, Wonder Woman, the Flash and Green Lantern terrorizing a group of homeless people. But it isn't the Justice League at all. It's actually Ultraman, Superwoman, Owlman, Johnny Quick and Power Ring. Fans of the "Justice League" comic will recognize them as the otherworldly counterparts to the Justice Leage - the Crime Syndicate. Somehow Max Lord and the Crime Syndicate are working together.
The Martian Manhunter appears from deep space, and after a brief (and obligatory) misunderstanding/fight scene, the Manhunter reveals that "your world is at an end," and tells the heroes about the OMACs under Max Lord's control.
The reviewer describes the script as "light and fluffy cartoon adventure stuff." It sounds like the filmmakers are doing everything they can to distance themselves from any existing Superman or Batman television or film project. This isn't going to be a movie for adults, but more of a live-action version of the Saturday morning cartoons.