Mainly, the addition of jet packs. Oh, yeah. Jet packs. In the Halo Reach beta (available through the Halo 3: ODST menu options) four different loadouts are available with super abilities: invisibility, shield, sprinting, and jet packin'.
Instead of picking up equipment as you did in Halo 3, you'll be granted use of those in a more permanent form — with a short cool-down period after each use. All are pretty fundamental to the way you play in Halo Reach multiplayer, but none more than the addition of flying around at will.
It's not as quick as launching upwards in the Tribes series or as easy to manage, but skyward zipping around in a Halo game is incredibly satisfying. Battles take to the air constantly along with your usual moveset of precise shooting and melee attacks. I've only played the last couple of days the beta has been live and already I've been involved in countless of epic aerial struggles.
Jet packs aren't an easy "win button," either. The drawbacks of the slow-to-start ignition and the fact that you're sticking out like a watch tower's light beam when you're using them helps keep the matches balanced. At least, it seems that way from what I've played so far.
For some, it may be worrisome that Halo's once "pure" gameplay has been infected with the seemingly viral inclusions of leveling up, loadouts, and perks to first-person shooters. But it looks like Bungie is taking the right path with respects to Reach. The changes are at the root of the new Halo experience but they are modest and avoid stitching in anything too crazy (i.e. thermonuclear rewards for masterful players). So you can rest easy, Halo fans. We're in for a good, high-flying ride this fall.