It's my belief that if we go straight into a Justice League movie, it'll look even more like an Avengers knock off than if they did the lead-in films. And really think about it, if this film bombs, it will bomb hard. No new DC Movies, (with the exception of Superman if MOS does well) for a loooooooooong time.
The key difference in my opinion is the way to go about this, first off DC Characters are different from Marvel's, no debates here. The only characters in the Justice League that are somewhat similar to a typical Marvel hero are Green Lantern and Flash (by personality , not by mythologies). But they should use that to their advantage by making each solo film their own epic and not to worry about the "shared universe" the only studio interference should be that they keep it all under control by casual mentions and not full blown cameos (make audiences unaware that a Justice League film is being planned).
The main difference between the Nolan films and the Marvel Studio solo movies is that the Marvel films feel like your standard superhero film which works for Iron Man and Cap, not so much with Thor and Hulk (I enjoyed Thor but I think is missed on the more epic sword and sorcery feel that could have separated from the other films). DC should avoid this and make their solo films their own ball game, have Wonder Woman be a tale of Greek Mythology coming into play in today's world with social-political overtones. Have Aquaman be the undersea adventure with an epic battle for the control of Atlantis against Ocean Master, and finally get rid of the jokes by making him the conflicted warrior king he was destined to be.
After that focus on Flash which is probably a safer bet than the other two but still keep it as it's own story. As Flash is more of a standard superhero it could make a great fun summer pop-corn movie with potential CSI elements in there. Now with Green Lantern, we have a bit of a problem on our hands- everyone hates him right now and with the approach I'm suggesting I'm not opposed to seeing him in Justice League where we see his "new" introduction but if WB are willing to give him another go- go full blown space opera here like Star Wars, galaxies away against Sinestro and his Corps fitting with the epic scope of the other films.
Onto the big two as I said, Superman is fine from what we see so far and if it's any indication his film will set the films for a JL film either way they go. As for Batman, Nolan's films are so beloved right now that I think it might be best to either save his film for last or introduce him again in Justice League (keeping his appeal as the mysterious "cool" one). On the other hand if they do decide to have another Bat-film than obviously make it different from Nolan's approach but have it in the loose shared universe we have going.
Finally Justice League it's self, now depending on the success of the hypothetical six films we have- if one or two don't do that well we can still get this one done. Anymore than that, we may be in trouble here but for the sake of argument let's say they've done well enough audience know and like the heroes and are surprised to see a JL film. Now of course the obvious answer is "make it big"- that goes without saying but in terms of tone (where the marketing comes in) it needs to be dark for most of the movie. Yeah, yeah I know that's cliche to say but hear me out.
The villain needs to seem larger than not only life, but larger than the heroes whomever it is it needs to actually seem like Earth is doomed this time- we need to make it look like it will be their first and last stand, an all around gloomy atmosphere where the audiences are at the edge of their seats. I'm talking a disaster/war movie. Market it as such, that humanity is in grave danger and these very different heroes, almost gods have to work together- but that they might not make it out in the end. But at the end, when the League do prevail it shouldn't look like the Avengers with the heroes kicking ass- it needs to be more like angels from heaven have stopped Satan from making Revelations become a reality- leave the audience in total awe and wonder. Instead of people coming out shouting "HOLY ****! THAT WAS AWESOME!" people should come out thinking "Wow..." utterly speechless.
