Thruster Packs are designed for the players with deftest footwork. They boost you a few feet back, sideways or forward, simultaneously popping the view out into third person so you're able to reorient yourself more easily. This trick is best thought of as a way of building on manoeuvres, rather than opening up whole new tactical possibilities a la Halo Reach's jetpack. Use it to confuse a sniper, or gain the shelter of a doorway a fraction of a second quicker, or close the distance to a backpedalling enemy.
Another nod to the Call of Duty series, Autosentries rain death across a 180 degree arc till they or their owners are destroyed. Use them to tie down chokepoints or expose enemy movements at long range. When defending a base alongside another player armed with an Autosentry, try deploying yours next to him and vice versa - foes will assume the player nearest the sentry is the owner and target him first in hopes of taking both threats out simultaneously.
One big weakness of the Autosentry is that it's an absolute sucker for holograms. Once you know this Armor Ability is in play, by all means send a puppet-version of yourself galloping under its nose. The little guy won't be able to resist.
It sounds like a sci-fi medipack, but the Regeneration Field is actually an oddball marriage of elements from Halo 3's Regenerator and Reach's vehicle-stopping Armor Lock. Like the former, it'll heal all players in the vicinity (including enemies). A good choice when everybody's holed up around a dropped flag, though don't depend on it - the Field won't protect you from additional damage and doesn't refill your health instantaneously.
As with Armor Lock, the Field emits a short range kinetic blast, shoving nearby foes out of the way; unlike Armor Lock, the blast is triggered on activation rather than deactivation, making this more of a fire-and-forget tool when used offensively. Throw it down in the face of a charging Sword-wielder to open a life-saving window of opportunity for your snipers.
Here are five thrilling tales of ineptitude and victory from Halo 4's Spartan Ops mode. Beware, the idiocy may rub off.