Frankly, I don't see why a build-up is even necessary. It's not like Star Wars had to have multiple movies first showing you adventures of Darth Vader, Han Solo, Luke Skywalker, Princess Leia, Chewbacca and others. It just went straight into the story introducing the audience to all those characters and where they were at in life. Same with Lord of the Rings. We didn't get some movies of Gandalf, Aragorn, Gimli, Legolas and others first. The movie just went into the story and introduced the characters to the audience.
Moreover, a Justice League movie already has two familiar characters in for the audience with Batman and Superman. Plus there is already name recognition for characters like Wonder Woman, The Flash, Green Lantern and even Aquaman. Shows like the Big Bang Theory mentioning and talking about those characters means that those names and characters are things people will have already heard of it however cursery. That little bit of knowledge may be even to have them see a Justice League to find out more of what these characters are or to see them in action.
The idea of making a movie for every character to bring them together for a movie is an interesting idea but it doesn't change the fact that those characters are still be introduced to a lot of people for the first time. I'd never read or really knew anything about Ironman before watching the first movie but I still went and enjoyed it. It didn't require a build-up of movies introducing all the characters in that movie either. Just do a well done story and the movie will be fine.
Personally, I've never seen the Thor Movies or The Incredible Hulk or Captain America. I actually didn't get around to watching The Avengers until it premiered on the Movie Channel and saw it then since I had no interest in most of the characters. In the end, it was an enjoyable flick of big silly action like I expected. It was fun but nothing I have a desire to watch again anytime soon. But it is also why I don't buy into the belief that DC has to introduce a majority of the Justice Leagers in their own seperate movies first. The Dark Knight Rises practically made the same amount of money as The Avengers and that was one single superhero with only two movies building up to it. I think it shows that having Batman in a Justice League movie should be enough for it to still be a huge success and get people to see such a film.
Maybe it doesn't make as money as The Avengers because there is less familiarity with the other characters so some moviegoers are hesitant. But again, as long as the script and story are solid enough, I don't think it will bomb. Frankly, if you go the trilogy route with the Justice League concept, then I don't see what the problem is with introducing all the characters and forming the league in the first movie. Even The Avengers spent a lot of time introducing the characters and have them meet up and fight each other before finally rallying together and becoming a team. Even with all those other movies, the movie didn't begin with The Avengers formed and in a team. They still had to do the origin of it.
If DC just started out with a Justice League movie and went from there, I don't see why it would be such a bad thing. From that introduction, you can then branch out and have the different league members star in their own movies. How is that a worse approach? That said, trying to jam things into MoS2 to speed up the process to a Justice League movie or out of fear that people need to see these characters in other movies first could end up doing more harm to the idea if done poorly. Right now, it is starting to sound like the result will be an Iron Man 2 of haphazardness. Moreover, the first Man of Steel was just ok but it had a few issue also. Instead of looking to refine that franchise and get it successful on its own, it is going to be this half step to a Justice League movie and where will that leave the Superman movies after it?