Though I'm not a huge gamer in general, about 30% of my waking time since Reach came out has been spent playing it.
Firstly, loadouts are available in multiplayer. You can now choose to play with one of five special powerups that you can activate every once in a while: Sprint, jet pack, armor lock, active camo/radar jammer, and hologram. The only one I really like using (or playing against) is Sprint; however, quite a few people like the others.
The battle rifle is now the DMR, and has longer range and an expanding reticle-if you fire too fast you lose accuracy.
Duals are not duals anymore. My least favorite change.
No more spike grenades. This was because, according to bungie, it was too much of a pain to switch between spike and plasma when they do roughly the same thing.
Health and shields now exist. Shields always regenerate; health only does if you're an elite, but there are healthpacks for spartans.
My favorite addition in Reach? Elite slayer. It includes the ability "evade" which allows you to dodge around 5 meters in whatever direction you want, and a few others, but evade is my favorite by far.
The maps are all very large, sadly. I'd say that there are about three Halo 3 maps smaller than the smallest Reach map. Playing with three or two people is a pain, even on the smallest maps.
Despite the few things I disliked, it's a great game overall, a lot of fun to play, and different enough from Halo 3 that it seems like a whole new game.
You can pick it up here: Halo Reach