Bungie has posted a detailed preview of multiplayer game mechanics in Halo: Reach, ahead of the game's beta test on 3rd May.
The principal new info concerns the new Armour Abilities. These are the persistent special skills that replace Halo 3's one-shot Equipment. You can only have one equipped at a time, but can use it as much as you like, although it takes time to recharge.
You'll select your Armour Ability as part of a Load Out at the beginning of each round or match and every time you respawn. These are playlist and game specific - you can't define your own - and set your Armour Ability and starting primary and secondary weapons.
The Armour Abilities themselves are detailed. Active Camo grants invisibility but is less effective the faster you move. Armour Lock makes you invulnerable, deflecting projectiles and destroying vehicles that touch you, but you're unable to move.
Evade gives you a quick jump in any direction and sheds locks and tracking. The Jet Pack shoots you into the air, either holding down the button for a sustained boost or feathering to float; you can aim and fire while flying, but you're an easy target. Sprint gives you a burst of speed, but means you can't use your weapons.
Some Abilities are specific to Spartans, and some to Elites.
Halo: Reach is due out this autumn exclusively for Xbox 360.