I saw this I AM LEGEND trailer at THE SIMPSONS: I'm going to guess it's the teaser trailer because it doesn't show you why Will Smith is not alone, or who's with him, which is what I AM LEGEND is really about, at least the novel.
Looks good, eh?
I wonder if the same sort of critters are still his antagonists...
I don't know if you've been following the progress of this movie, at all, but that's almost as interesting as the movie itself. It's been gestating in development for literally decades. Arnold Schwarzenegger was originally attached to it, when he was in his prime with Ridley Scott set to direct at one point. I'm guessing Nicolas Cage was attached at one point.
Often the first trailer shows much less than the regular trailer. It might be because they don't have the footage, but it's also because they don't want the movie to feel old. JURASSIC PARK had a great teaser trailer.
Speaking of the difference between teasers and trailers, I generally like teasers better. They give away less and seem to try and set mood more than trailers, which seem to just try and show as much footage as possible. There seems to be more creativity involved in teasing an audience. I guess it's kind of like the difference between seducing someone and dryhumping them on the couch while you play tonsil hockey. The ultimate goal is to get them in the sack, but there are different ways to go about it. Robert Zemeckis apparently thinks you need to whip it out and show your potential partner all you've got. I disagree.
Part of my job is watching trailers, so I watch A LOT of them, usually again and again. So I've got a lot of opinions on them.