Before I start listing the reasons I think it wouldn't work, let me say that I REALLY hope this movie does good and is every bit as good as Avengers. That being said, the reason why I don't think it will work is because DC Characters are separated from each other
Let me explain. In the comics, how often do you really get the sense that these people are in the same universe? I don't mean books like Justice League or Teen Titans, or any of the team books, I'm talking solo series. Superman is fully capable of dealing with just about anything without other heroes. Batman has his own corner of the DCU that is practically a universe in itself. The Green Lantern Corps could exist in a universe without other heroes in it and it wouldn't really change anything. Wonder Woman could do the same, and so could Flash. Now to me, doing a shared Marvel Movie Universe will always make more sense that doing a shared DC Movie Universe. The reason is because Marvel was made that way and DC wasn't. Stan Lee really was the first person to really give the sense of a comic book UNIVERSE. He had characters meet each other, outside team books, and slowly all those interactions formed the Marvel Universe we know today. Even with the early team books of DC, they weren't really connected. JSA didn't have Superman or Batman, or even really reference them. Not until the Silver Age did DC start trying to make a shared universe for their heroes to be in. However, that feeling of isolation is still there even to this day. Don't get me wrong, I love DC Comics, but I can also see this as a very real issue. Now, if DC simply wanted to make a few more movies and THEN have a Justice League movie, then fine. The reason is that it's not like the Avengers where most if not all the characters are unknown. The reason they needed to build that movie up is because of the fact that over half of the average movie going public who had never read a comic or watched a cartoon about these characters had no idea who they are aside from the Hulk because of his previous 2003 movie and Bill Bixby/Lou Ferrigno TV show. People know who Superman is, they know who Batman is, they know Wonder Woman, and even Green Lantern at this point(Sure the movie bombed, but it's still in the public knowledge). I want the movie to be good, I'm excited that it's being made, I'm just very skeptical as to the how DC is handling this.