Players control a Spartan named Noble 6, part of the Noble Team. The big first for the Halo series will be an entire team of Spartans fighting the alien Covenant at the same time.
This is a prequel to Halo: Combat Evolved, and so Bungie wants the Covenant to feel dangerous again, like they did in the first game. They no longer speak in English, there's a new enemy type called Skirmishers, who attack in packs, and the Elites are back as foes.
Health packs are back, combined with regenerating health as in the original game.
More stuff happening at once: Bungie says it can now cram 40 enemies and 20 vehicles in an area at once.
New weapons: The Designated Marksman Rifle sits somewhere between sniper and battle rifle and the Needle Rifle acts like a Needler but with more accuracy and flexibility.
Sniper rifle, assault rifle and magnum pistol return, as do frag and sticky grenades. Spike and flame grenades are out.
From Bungie.net :
Planet: Reach (excerpt from CAA FACT BOOK [l.update 12.7.2535])
* Length of day: 27 hours
* Length of year: 390 days (local)
* Gravity: ~1.08 g
* Natural satellites: [2] Csodaszarvas, Turul
Reach is the 4th largest planet in the Epsilon Eridani [K2 Orange-red dwarf] system, second closest to ε Eridani. Stable for planet of its age [<1B years]. First planetary confirmations were in early 21st century.
Reach is the location of the UNSC's largest and most active shipyard. It is also where the UNSC's most elite troops, the Orbital Drop Shock Troopers, are trained. Reach is also the largest non-automated exporter of titanium. ε Eridani's superior interstellar jump point has been the most active S-F space transfer zone (arrivals and departures) for over thirty years, with ε Eridani's inferior IJP and Sol's superior IJP regularly trading second and third place spots.
Sorry, but this is all I could find right now.