Ok, Bungie is pretty notorious about leaving hints in their media about the plot and features in their games. The commercial seems to continue this tradition. For example, they focus in on MC putting a gun on his back--which we now know will be part of the game (i.e. you can see weapons that people carry).
So lets list things that MAY be hints. Don't be afraid to mention stuff, sometimes it gets other peeps juices flowing.
1. MC put his weapon on his back. (Confirmed) You can now see peeps weapons that they carry on their back.
2. Helmet on the ground. The closeup surely is a "hint" at the bonus pack with the helmet.
3. Shield grenade. Bungie has already confirmed at least 1 new grenade. Is this another?
4. MC sprints toward the battlefield. The X button does SOMETHING new. Bungie staff mentioned wanting to use it a LOT in the recent pre-Alpha with 1UP, which would mesh well with sprint.
5. MC jumps (sprint jump?) and appears to do a flying kick off the cliff. A new melee attack to go along with sprint?
6. At the center of the bad guys is one with a red piece of cloathing and he has a large sledge hammer. Possible boss character?
7. Massive battlefields. Air and ground combat at once on a large scale, including air born bombs. A lot of dead humans on the ground. 13 mile view distance for a reason...
Btw, 720p version well encoded here:
http://xboxmovies.teamxbox.com/xbox-360-hires/4001/Halo-3-Trailer-HD/
8. The boy is probaly John (John117), that is Master Chief. The girl is probably Lisa or Kelly (childhood friend). Possible indication of other Spartans in Halo 3?
9. Some spotted a mongoose at the very end.
Any others people spotted?