The deal with a JL movie is that it would be even riskier than the Avengers, because it would involve more far more fantastical, sci-fi elements. More egos to bring together and a complicated story to merge.
The DC characters just have more of an epic scope to them. I remember Johns saying once that "our characters are bigger than theirs." When I read that I didn't take it as DC > Marvel, but that the Marvel universe is simpler and has less of a grand, epic tone to it. Okay, Marvel has Thor, but the whole point of Thor is a "god" becomes a superhero by learning humility and the concept of living as a human. DC characters are already up on more of a pedestal. Even the grounded-in-reality Batman of the Nolan-verse is "our watchful protector, a Dark Knight." Iron Man, and Tony Stark for that matter, is just another superhero and celebrity in the world.
That being said, DCE still fails to see the need to flush out other stories away from Superman and Batman. Lets just hope GL doesn't tank.