1. The FOV is 78 degrees in Reach, Halo 3 was about 60 degrees.
2. At the basic level, you move slower than the Chief (not including Sprint).
3. Jump heights are "a little more realistic." It's a little more grounded.
4. Lunge range is shorter than it was in Halo 3, but it's not really noticeable.
5. It takes two melees to kill a player: one to take down the shields, the other to kill.
6. You are not invulnerable when carrying out an assassination animation - you can still be killed.
7. Five headshots to kill someone with the DMR, five headshots to kill someone with the magnum. Differences between the guns are mainly rate of fire, accuracy, and range. The DMR fires more slowly but it can reach you from farther away. The range of the magnum "plays right outside ideally AR range": magnum a very skilled user can take down a guy with an AR.
8. Frags are more dangerous in Reach, they're more scattered on the map. Less grenade spam than in Halo 3. Larger blast radius.
9. As others have mentioned, you can shoot grenades and have them explode.
10. The grenade arc in Reach is different from Halo 3. It's not that much of a difference, but "you're going to have to relearn some stuff." If you're sprinting and you throw a grenade, you'll throw it farther.
11. Secondary fire on the grenade launcher
12. You "gotta be trying to hurt yourself" to receive fall damage in Reach. The stun is back if you hit the ground hard.
13. For health and shields, it'll take a longer time than in Halo 3 for them to recharge. Health also recharges, but it recharges "at thresholds." If you're about to die, your health is going to regenerate so that a single scratch is not going to kill you, but you'll still have to go out and search for health packs.
14. Health and shields on Elites and Spartans are different. Elites are larger, so they have more health and more shields. The time it takes to kill an Elite with the AR on the open battlefield is very close to that of a Spartan. It takes 6 DMR headshots to kill an Elite.
15. The beta will only feature on Spartan Spartan whereas the final product may feature on Elite Elite as well.
16. Player encounters in REACH will be longer than it was in Halo 3.
17. Almost everything that is coming back from Halo 3 has been tweaked in some way.
18. There have been changes made to the sword combat.
19. In regards to sandbox changes, there is no "uber weapon." There is no particular load-out that dominates all the rest.
20. As I suspected, there are no Spikers or SMGs in Reach.
21. Aim Assist is on a "for weapon setting."
22. Sniper is "even more of a weapon for adults."