It's not so much about exposition as about the audience liking the character versions in that particular approach. They didn't do an Avengers film and then Iron Man, Thor, Captain America... they did those individual standalone films first and then after the audience liked these films they did the crossover.
True. But then again, you could say that audiences might like the depiction of the JLA characters in this movie without seeing their standalone movies. Assuming the movie handles them well - makes them well written and has them cast as well as the Marvel movies did.
And indeed, it should be remembered that Mark Ruffalo's Hulk/ Banner was by-and-large much more positively received than Ed Norton's take and The Incredible Hulk movie.
I can see both sides of the argument and I can see how the movie could go either way. We won't know until we see it - right now, I just want to hope for the best.