I don't understand the argument that a JL film will be incoherent without standalone films preceding it. Did people watch the Dirty Dozen utterly confused as to the backstory of every single prisoner? What about Star Trek?
People say The Avenger's worked with this model, but obviously the film relied on many who had not seen all of the preceding films, based on the box office take alone. The Avengers itself works as a story without having seen all of them either. As it always does, the film working depends on how well it's told. We can have a bunch of standalone films, but if the JL film is crap, it won't matter if they have been introduced or not. What about the Justice League animated series? It did practically the same thing, to great success.
I think the reason it's not the same as Dirty Dozen, or any movie with a large cast for that matter, is because each character in Avengers/JL, just by looking at them and their crazy abilities, raises a question about their uniqueness. If one of the Dirty Dozen could run at the speed of Light, you'd want to know why the hell he was doing that.
Not necessarily questions about their personality, but questions regarding why this Flash guy is super fast, why Wonder Woman is super strong, etc.
Since we have Themyscira, Krypton, etc, thrown in the mix it's not even as simple as X-Men where the explanation for every character's powers is 'mutants'.
That being said. Superman, Batman, and Green Lantern are already taken care of.
For Flash you'd only need
A: a 15 second explanation (like how Coulson explains why Banner is the way he is in the plane to Cap)
or B: a 5 minute flashback/montage (like Dr. Manhattan)
For WW the same thing, and possibly option C: we get a 20 minute WW origin. Perhaps Supes/Bats can go to Themyscira and it can be a set-piece. So many options.
I don't see what the big deal is with this overloaded origins business.