It's fantastic to see The Avengers being made in to a film, but I have massive reservations. When the X Men films were being made it seemed as though the process to give all of the many characters a decent story line was extremely difficult. The Avengers will have this same issue.
As far as the villain is concerned, if it just Loki, I think the Red Skull is involved as well, I'm very dissapointed. To bring 'Earths Mightiest Heroes' together demands a huge threat. We should be looking at Thanos or Galactus.
I hope I'm wrong, I really do.