This is from the EGM multiplayer preview:

Spartan Laser: Big directed-energy weapon designed for destroying vehicles. Wipes out a Warthog and it's passengers in one shot. Is unguided and has a charge-up time before it can fire.

Brute Spike gun: SMG-like weapon that fires superheated metal spikes. Has knife mounted.

rebuilt Assault rifle: actually useful at intended ranges.

Spike grenade: Sticks into everything, uses a shaped-charge detonation to destroy heavy vehicles. (downside: only 2 of each type of grenade now, bringing it from 8 to 6)

No more cliff-wall area borders. If you walk too far out of the combat area, you get lit up by autoturrets.

The Mongoose. With a second player riding brokeback.

The "Man Cannon", a device that launches players halfway across the map.

Even more customization options, and the ability to set in stone rules that were player-honesty based in previous games. And post them into the rotation of the player-matching service.