From the bottom of my heart, I truly believe that a Justice League movie could work. I really do. That being said, I don't think that we will be seeing a Justice League movie anytime soon.
Let me just start out bu stating what has already been stated many times before me: Nolan's Batman wouldn't work in a Justice League movie. Though his Batman is fantastic, I just can't see him in a movie where he interacts with someone like the Green Lantern or the Martian Manhunter.
I think that in order to include Batman in the Justice League, Warner Bros. will have to reinvent the Batman franchise first, which they already said they are going to do anyway.
Now I honestly have no clue if this new Superman would work in a Justice League movie. I've read that Nolan and Snyder plan to keep him in his own seperate franchise, but you never know. I would love for Superman to be a jumping off point for a Justice League movie, but at this point in time, I really can't say if it would work or not.
With the Green Lantern, I liked the fact that the recent movie didn't really contain references to other superheroes. If Green Lantern is going to be used as a jumping off point for a JLA movie, I would rather them wait to do it in the sequel so audiences will be more familiar with the Green Lantern before having to worry about all these other superheroes.
The way that Marvel has set up for the Avengers has been, in my opinion, genius. I would enjoy if the DC movies had post credit scenes that set up for the next movie, but at the same time, I don't want DC to completely rip off Marvel.
When it comes to introducing characters in their own movies or waiting until the Justice League to introduce them, I'm a little torn. Obviously you have characters like Superman, Batman, Green Lantern, and the Flash that deserve their own movie. But their are some characters that I'm not sure about. Martian Manhunter is a big one. I know he is an important character, but I don't know if he deserves his own movie. Other characters like Aquaman or Wonder Woman can support their own movie, I just don't know if I personally would want to see it.
If a Justice League movie was to go into production tomorrow, I wouldn't be very excited. I think that Warner Bros. has to wait and build up their characters first before putting them in a movie together.