Some things that I've noticed after playing the demo about 15 times:
1. Enemies will sometimes jump or fall into the water. After swimming for five seconds they suddenly die. They do not attempt to go back to shore.
2. If you're shooting an enemy from a long distance away using the standard rifles (not snipers), it takes literally around 30 body shots to kill a single person. A single head shot from any distance kills.
3. Enemies don't react to certain parts of their body being hit.
4. Head shots from helmeted soldiers knock off their helmet. The enemy is neither phased nor goes to pick up their helmet. Shooting a helmet wearing soldier in the neck area still kills them in one shot though.
5. AI does not work as a team. They may approach you in a group, but this is a result of each enemy individually approaching you, rather than first forming a team and then approaching you.
6. AI view distance is very short compared to you. In an empty field with nothing between you and the enemy, you can see the enemy WAY before they register you in their sights.
7. Enemies stand around things that are clearly about to explode, and make no move to seek cover or gain distance away from the impending explosion.
8. The destroyable environment is limited to foliage, wildlife, certain building panels, and vehicles. Small miscellaneous objects can't be destroyed, only moved. Things like sandbags can't be moved.
9. You fall very very slowly in relation to other things. If you take the helicopter up and bail out of it, the helicopter plummets to the ground way way way before you hit. I wouldn't be surprised if your sprint speed mode was actually faster than your free fall. If you land in water, you'll survive with no problem (must be somewhat deep).
10. You can't shoot through non-destructible things, like a thin wall panel that hasn't been coded to be destructible.
11. The gun boats have incredibly unrealistic view distances, accuracy, and the ability to find you in foliage.
12. Enemy speech is tied to whether an enemy is alive or dead, not knocked out. If you knock out the three guys chatting around the rock on the beach with your darts their conversation will continue, despite all three of them being knocked out.
13. Your aim with the sniper scope is a lot steadier on a heavy sniper rifle than it is with the same sniper scope mounted on one of the standard rifles.
14. The sound for the silenced pistol should be a dull thud, not the standard imaginary Hollywood movie-silencer sound.
15. There are sharks.
16. If you go outside the bounds of the island or mission area you will be vaporized.
17. Vehicle damage is not area-specific. You can repeatedly shoot just the minigun on a vehicle and the entire thing will end up exploding. If you take two shots from a sniper rifle and take out the driver and gunner, the entire vehicle health will be down to around 60%.
18. You cannot ram enemy gun boats with your own gun boat to kill the enemies in the gunboat. Even if you land on top of their gunboat.
19. You cannot climb into occupied gunboats, so if you're out in the open water with a gunboat chasing you, there is no way for you to destroy it unless you get back onto land first so you can pull your gun out.
20. Enemies can see underneath their gun boat. If you swim underneath them they will start shooting.
21. If you get shot and your suit power reaches zero, you lose the ability to breathe underwater.
That's all I can think of for now.