The problem is that DC's universe and the greater American audience do not blend well. They have a hard time expressing their heroes without making them seem cheesy or they take things to a whole new level like with Nolan's Batman, or they create a Superman who has to grow from his first movie, before he can settle into something bigger. (In my opinon, he's still not 'Superman')
Frankly, skipping ahead to Batman+Superman movie is exciting, but ill-thought out. They should do one more Superman movie, so we can at least see that Superman is actually a Superhero, not just an Alien who just found out who he was and where he came from. Still some development necessary.
Then on the other end is Batman, if they are going to separate this from Nolan's Batman, which I'm okay with, they should incorporate him some other way first before the Superman+Batman movie.
When you think about it, Superman has done little so far to be someone in which other Superheroes will just willingly follow. We need to see him prove himself, and then maybe we can move forward.
In terms of introducing the other characters, Flash, Green Lantern, Wonder Woman, Martian Manhunter and Aquaman do not all need movies. Although, in my opinion certain character will need it for the Justice League to really 'sell' to audiences. Aquaman for instance, his royal attitude would be great for interactions among the group. The Flash's demeanor as well, not to mention Wonder Woman's personality. From those three the audience will probably connect the best with The Flash and Wonder Woman. Aquaman would be the hardest to pull off, but I think he's the most necessary one to have to succeed.
The way I imagine it, going Batman-Supes all TDKR like, its too early for that. Introduce a new Batman for this arc of movies, maybe as a cameo with Martian Manhunter in a movie. Maybe even do a Wonderwoman/Aquaman movie, where they have some sort of struggle against one another, nothing new there, but throw in the Superman cameo at the end to unite them under a greater cause. These would really give decent backstory for a JLA movie.
Marvel did Hulk and Ironman in 2008, Iron Man 2 in 2010, Captain America and Thor in 2011. We got a taste of S.H.I.E.L.D., Nick Fury, Black Widow and Hawkeye between those movies. Finally 2012 gave us the Avengers...
All DC has for it on it's plate right now is MoS from this year 2013. They would need to fill in 6 more characters between now and 2017. Their current time line is:
MoS 2013, MoS/Batman 2 2015, The Flash 2016 and JLA 2017 (according to rumors). I don't see this happening that way so well, basically introducing 3 heroes of a 7 hero team, the audience is going to feel detatched from the other four heroes. Maybe it will generate the money, but I don't think it will do it justice.
The good thing is, I'm not as emotionally attached to DC as much as I am with Marvel, and my expectations are low. And yes, my thoughts are all over the place, sorry about that.