I won't link the clip I saw because I have no doubt it will get taken down before too long and I don't feel like continuously searching to find another one to replace it. If you don't plan on seeing The Hunger Games this weekend, and don't want to wait until Monday when it premieres online, just go to the Breaking Dawn Part 2 forum on IMDB and fish through the links all those people keep posting. That's how I found one that was still active, tbh.
To spoil the teaser...Basically it starts with Bella's typical voice over saying something like "after 18 years I finally know I can shine." She holds her hand up in the air and you see what the world looks like to her as a vampire now. You see her running through the woods all fast, you see Jacob saying he didn't expect her to seem so normal, and Edward holds her and is like, "we're the same temperature now." You see Bella's red, vampire eyes and the end of it shows a deer walking through the woods and she pops out from behind a tree. Basically if you read the book you understand what is going on with the deer. If you didn't, Edward is teaching her how to hunt.
That's it, guys. When they said it was just going to be a teaser they weren't kidding. The only comment I really have and I said it after I saw Breaking Dawn Part 1 is that Bill Condon must not have liked the fact that Stephenie Meyer's vampires sparkled because they do not in his Twilight movies. In BD1 they were on a tropical island with Edward in swim suits, tank tops, and shorts and he never sparkled in the sun. In this teaser, Bella is holding her hand up in the sun and Edward's skin is exposed and they don't sparkle. I remember watching New Moon and thinking it was ridiculous how much Edward sparkled. If he walked past a window that had light coming in he looked like a fucking crystal statue. Maybe it's just because I haven't seen a high quality version of it yet, but the inconsistency is annoying.