It would be great to have a truly destructable environment. Say you're about to enter a building but think the door is booby-trapped, you just smash a window and climb through.
If an enemy shoots at the wall you're behind, fragments should fly in the air.
Tanks and explosions should shake the ground, like in Ghost Recon.
Almost forgot lol, would be nice to be able to climb over things as you can in Cod 4.