Armour Abilities
The single largest unique new features to happen in Halo: Reach are without a doubt the armour abilities. These are abilities that are governed not only by your class selection, but your species. So far ((Bearing in mind that we're only in Beta stage at this time)), the Spartans have 4 armour abilities, and the Covenant only have 1. The abilities all have their very own specific advantages.
Note: None of the abilities, Spartan and Elite alike, can be used while tending to objectives. E.g, you cannot fly away using your jetpack or cloak yourself while in possesion of a flag or the oddball.
 Spartan Abilities
The Spartan's have 4 abilities to chose from when they spawn. Depending on the gametype, but the 4 are Sprint, Active Camouflage, Armour Lock and Jetpack.
Doing exactly what you'd expect it to do, pressing LB will give your spartan a temporary speed boost, allowing you to rush for those all-important weapons and objectives during gameplay. Naturally, you cannot shoot while you are sprinting.
 Armour Lock
Armour Lock is the ability to make your Spartan momentarily invulnerable. Holding the Left Bumper will cause your Spartan to drop to his knees in a ground pounding position. His armour will lock into place and you will be invincible. This is primarily a team-player's ability, as being invulnerable leaves you vulnerable when you come out of your armour lock. You can't do anything while armour lock is activated, but if you hold it until the end of it's charge, and EMP burst will explode out from your armour, leaving anybody in close proximity with no shield, giving you the chance to capitalise on your situation and take them out.
Probably the most universally useful Armour Ability in the lineup, the Jetpack allows the user to quickly zip up and down, in and out of cover. Tricky weapon spawns that would otherwise be hazardous, like the Rockets on Powerhouse are turned into a grab and run job with the jetpack, allonwing you to nab weapons from tricky places and escape unscathed. They give you the chance to get away from a pursuer, or get the jump on one, giving you a sneaky assassination whenever the opportunity arises. The momentum when you use the jetpack is exactly how you'd imagine it to be. It's easy to launch yourself up, but not so easy to slow yourself and go up in the same Jetpack charge. Overall a very useful ability.
 Active CamouflageRadar Jammer from Halo 3 also. A device that was thrown down and disrupted the enemies radar by showing up multiple enemy blips to hide yourself in in the chaos on the enemies radar. Both these aspects make up the Active Camo in Halo: Reach. When you press LB, you are cloaked with Active Camo, leaving you completely invisible when you stand still, and the enemies radar is filled with enemy blips in your general location. The more still you stand, the slower the camouflage runs down, allowing you to get from 30 seconds to a minute out of it if you stand completely still. As useful as this may seem, it's also quite counter productive in Halo: Reach, as even when an ally uses it, the radar still shows enemy blips and not friendly ones. Tight teamwork and lots of communication are the only way around this snag.
 Covenant Abilities
 Evade Roll
The Covenant's Evade Roll is an ability that Halo fans have wanted to see accessible in multiplayer for a long time. Pressing LB will launch your Elite in whatever direction you have the Left analogue stick held towards, giving you a sudden burst of speed in that direction, launching you out of harms way of anything being fired at you. It's great for evading anything from explosives to oncoming vehicles. The player can use this ability twice in quick succession before having to wait for it to recharge.