Bungie has said this pre-rendered trailer features in-game Halo 3 models and allures to new game features. The video is also available in HD on Xbox Live Marketplace.
Halo 3 things we can derive from the trailer:
1. Master Chief is a badass.
2. Chief Junior is “shown” in the flashback opener.
3. One of the new weapons is a “force shield grenade”.
4. The X button might be used to “sprint” in battle.
5. Brutes will have energy blades & hammer weapons.
6. At the end the new Wraiths are shown.