It's not that it won't work, but that WB will not commit to a JLA movie due to the fact that they don't really pay attention to nor invest heavily on superhero genre movies except for Batman, and I doubt they would make an except for JLA even though it has Batman in it. However, if The Avengers crack a billion WW in the box office, WB might be tempted to give it a shot.
Without a doubt, JLA is more powerful than TA because of the presence of Superman, WW, GL, and Martian Manhunter. But I always feel that TA is more of a team than JLA because of it, and members of the Avengers do compensate for each other's weakness. In JLA, Superman pretty much has no weakness except for that green rock.