The thing people seem to be missing is that team movies have been done before Avengers and will be done after Avengers without separate intro movies for the team members. It's just that most other team movies don't portray super powers.
Hell, the "gather the team" storyline is a tradition in some genres like westerns. I think DC can make it work.
Good point. The Seven Samurai, The Dirty Dozen, Ocean's Eleven
-- there are tons of movies about assembling a team of protagonists the audience has no prior familiarity with. There are even superhero examples, like Mystery Men
and The Incredibles
But then again it needs rock solid writing and by the DC track record (excluding the Nolan Batman movies) i'm not so sure.
Additionally it will inevitably be compared to the Marvel movies and their approach.. might even lead to more problems with the production when people start to ask wo the f.. Martian Manhunter or Wonder Woman is and why they didn't get their own movie before the Justice League.