Avengers isn't successful due to the shared universe, in fact given the story the shared universe is an inconsequential element, it's successful because it's a good fun ride that's also a bit of a novelty, that's it. People aren't going because it's the culmination of this grand universe, they're going because it's a bunch of well known superheroes joining forces and battling aliens and giving the audience an awesome show. It doesn't matter that Loki is the villain or the Teseract was from Cap America, or that Shield are the government agency, all that matters is Iron Man, Thor, Hulk, and Cap are sharing screen time and the film doesn't try to be anything more than that, and it does it fantastically.