The problem is, DC is WAY behind Marvel in terms of film development. Marvel has been cranking out superhero movies constantly since 1998 (Blade). Admittedly, a lot of them sucked, but they have gotten their franchises out there and really led the way in making superhero movies. DC, on the other hand mostly has just focused on Batman in recent years. They tried Superman again, but it didn't go well. They tried Green Lantern, and it was pretty awful. Ten years ago, when Marvel was putting out Hulk, DD, & FF Green Lantern would have been acceptable, but it came out the same summer as Thor, Cap, and X-Men: First Class. Those three movies were no Dark Knight or even Iron Man, but they were way better than the stuff marvel was putting out 8-10 years ago.
Marvel has paid their dues, made some crappy movies, learned from their mistakes and are now making solid movies (besides some of the movies being made by other studios like Ghost Rider: Spirit of Vengeance). WB/DC put all their eggs in Batman's basket, made a brilliant trilogy, but now are struggling to know where to go.
I think they need to forget the JLA movie idea for now and just concentrate on making some good stand alone movies that play to their characters' strengths. Make a Flash movie that's a crime drama. Make a good Green Lantern movie that's a space opera. Make a Wonder Woman movie that's a mythical fantasy and in which Wonder Woman doesn't wear the American Flag as a bikini. Hire good writers, good actors, good directors, and make some good movies, don't rush the process, and don't try to make everything dark! Gain some victories, make some mistakes, learn from both, THEN start thinking about how to make a JLA movie.