I dunno the exact makeup of the opening, of course, but some things that Whedon will probably need to do at the start:
*introduce the heroes and their backstories. Whedon and Feige have repeatedly stated that you do *not* have to watch the previous 5 movies to understand the Avengers, so that indicates that they're going to have to rehash them in the opening minutes. Annoying to those of us who already know the backstory, yeah, but it'll be necessary.
*open with a bang. Which means not opening with a bunch of exposition and "boring stuff." That may sound like it runs counter to my previous point: i.e., how do you show backstory without exposition, without turning it into a boring monologue of a character delivering their resume to the audience?
I *do* think the opening 15 minutes or so will just show Avengers, though, not Loki. The movie is about them, not him. I honestly don't expect to see Loki show up as a threat until about the 15 minute mark, after we've already spent some quality time with at least one or two of the Avengers. I'd say Cap would be the most likely Avenger we'll see first, then Tony, then Widow, then Banner, then Hawkeye....Thor will come much later, after Loki's already hit the scene.