Quick and dirty summary of the sandbox discussion, please correct me if I heard or interpreted wrongly:
Compared to Halo 3, Reach FOV is wider, camera is lower (these Spartans are shorter than the Chief), movement speed is slightly slower (although the effect is offset by the camera height and FOV, and you still move faster than a real-world human athlete), jump height is lowered, melee lunge is shorter.
Two melees to beatdown (first pops the shield). If someone's shields are down, if you hit him, he will die.
As mentioned before, you are vulnerable when "showboating" with an execution. Interrupting someone's execution will be rewarded somehow.
It takes five shots to kill someone with a DMR (four to take down shields, and then a fifth headshot will kill, otherwise three further body shots are required to kill).
The magnum is exactly the same (it fires the same round), though the accuracy and rate of fire is obviously different. DMR fires slower, but is more accurate, especially at range.
A "very skilled" player with a magnum can beat a guy with an AR "quite consistently" toward the outside of the AR's range.
Rhythm is important to maintaing accuracy.
You will have fewer frag grenades (they are sparser on maps, urk says they "almost become a power weapon", in that you go looking for them), but they are more powerful, and have a "significantly larger" radius. They have a damage curve along that radius, compared to the "fall-off" they have in Halo 3.
Currently, although it may be subject to change, you can shoot dormant grenades and have them explode (they give an example of sniping a guy, then sniping the grenades he drops when his buddy runs by).
The throwing arc is closer to Halo 3 than to ODST (thank God).
The new grenade launcher has two "modes" (on the same trigger). If you fire normally, it acts like a regular frag grenade, more or less (it blows up directly if you hit someone, of course). If you fire and hold down the trigger, the grenade goes out into the world and stays cooked until you release the trigger, when it then explodes. This is very effective against things in the air, as this secondary fire is an EMP explosion, and can drop banshees (or jetpacking guys).
Fall damage is in currently, but to a smaller degree. It also stuns you if you hit hard.
The time before your shield starts to recharge is longer than Halo 3.
Your health recharges, but to thresholds (like Far Cry 2, for instance?).
They have "embraced" the differences between Spartans and Elites. Elites are larger in size (and so easier to hit), but have more health and more shields. The time-to-death, however, is very similar on the whole, especially for the AR. The DMR requires an additional shot to kill.
Elites exist only in certain modes. In the Arena in the beta, there will only be Spartan vs Spartan; in the full game there may be Elite vs Elite. There will never be Spartan vs Elite in the Arena.
Individual encounters between players are intentionally longer (people who already complain about Halo's encounters being too long are going to love this!).
The magnum is apparently the weapon that has changed most significantly compared to Halo 3 (not ODST). Sword combat is also seeing an overhaul.
There will be gametypes (playlists?) where armour abilities do not feature. "Halo is all about options." So you can probably switch them off in custom games too.
The most obvious change to the sandbox is from equipment to armour abilities, but the most significant is the "focusing" of the sandbox, so that weapons and abilities are more situational and specific in their roles, and no one thing dominates.
Aim assist is set "per weapon"; smaller reticles indicate accuracy-based weapons. You get "not a lot of help" with the sniper rifle, and it is "even more a weapon for adults".