Looking forward very much to a demo.
Tall grass should help troops that stop moving, a little. Area fire should still be as effective as it would be if the ground were completely open, but any aimed fire bonuses to firepower ought to be reduced once target infantry stop moving and "take cover" or become pinned.
To get the effect we're used to seeing in CMBB/AK where infantry can go to ground in the open and, with a bit of range, ultimately stay there until the shooter (rifles/lmg) runs out of ammo, you'd need them to be in micro-terrain. For anyone who doesn't know what that means, it is the current term in vogue the military uses for dips and lumps in the ground under a few feet of difference.
CMBB/AK assumes all open terrain is chock-full of micro terrain.