This was asked at fanfest.

They said they it would take a lot of dev time to go over all the maps and assign how to use each bit of cover. They also said they didn't want it to play like Gears of War, not sure what that comment meant since i haven't played that series.

Killzone 3 only has cover system in single player modes. I never understood why, maybe somebody will point out the reason.

I suggest rather then sticking to a piece of cover, have the O button stick the player to current location while crouched.
Then the player could....
strafe left to lean left
strafe right to lean right
forward to un-crouch while held
hit O button to return to a normal crouch.
This could be set to lean or un-crouch based on how far stick is moved, so you don't have to stand completly if the waepon clears at halfway to un-crounch.

Maybe back could make the player duck more then a normal crouch? Not sure if it would look right on models to duck any lower without going prone.

This would require the animation and code but not the designating of all the cover in game, greatly reducing dev time needed. It would also allow crouching and popping up to use dips in rolling ground as cover or crouching behind a heavy that is being remote reped for cover.

This would let somebody kneel and pretend they are around a corner and lean in the open, but I suppose somebody could do that in real life too. Mime hiding behind his invisible box will get get shot in either case.