There's already one mediocre example of a Justice League movie, although only made-for-TV:http://www.youtube.com/watch?v=nz97bXR6H_0
Which goes to show that getting superheroes right onscreen is hard. Getting the atmosphere right, finding that balance between too mundane and too over-the-top, getting the audience to willingly suspend disbelief, all of that is incredibly difficult. It's been done wrong many times, and it's worked or even just mostly-worked only a few times.
Some fans of the Smallville tv show had high hopes for the proto-JLA that appeared in that series. But I agree that pulling off a JLA movie at this point seems like a longshot, especially trying to base it on the DC movies that have come out so far. Whoever can do it successfully will be hailed as a creative genius.