I wasn't aware this was significantly different; where could I go to find out more about these differences? I never liked the way Halo plays, but if this is different I may be interested, because I don't feel like buying Call of Duty again.
Hard to say. My impressions of the game came from playing the beta quite a bit. The number one change is the addition of armor abilities. Basically, in your average match, every time you spawn, you have the option of picking an armor ability to spawn with. The armor abilities are Jetpack, Sprint, Camo, Armor Lock, Hologram, Drop Shield, and Evade.
In typical matches, you won't have Evade because it's usually an Elite-only ability. It's basically their dodge rolling from all the single-player campaigns, only you can do it. It's very useful as you can close wide distances very quickly, and also use it to execute evasive or tactical maneuvers because you get two evades with your armor ability charge.
Jetpack is pretty much how it sounds, it's probably the best application of them in a multiplayer game since the Tribes games. Sprint is self-explanatory. Camo replaces the active camouflage pickup from previous games, so basically a cloaking device. Drop Shield replaces the bubble shield pickup from the previous games, adding the ability to heal any allies inside the shield, only the Drop Shield has a limited number of "hit points" so if it takes enough damage, it will go down. Armor Lock freezes you in place for a number of seconds (not sure on the exact amount), and for the time you're in armor lock, you're completely invincible. Upon coming out of the armor lock, it will release an EMP blast that will take down the shields of anybody close enough to you. The obvious disadvantage to this is that people can pretty much just chuck grenades at the right time, and you'll die right as you come out. Usually want some backup. Hologram sends an intangible copy of you Spartan running either in a straight line, or towards a point that you set previously. Good for distraction and flanking.
Now keep in mind that before, Halo had pretty much nothing going for it anything like this. Yeah, it had camo and bubble shield, but they were pickups. In Reach, EVERYBODY starts with one of these abilities, and has it the WHOLE match. The entire dynamic of encounters in the game plays out differently than it did in the older games, because everybody has a new way of fighting.