Idea taken from this thread and given it's own, because I'm important like that.
Realistic infantry suppression can actually be done with simple math and a "Courage" bar added to the player, right under the health and stamina bars. Every second it goes up 1 point. Every time a bullet or explosion lands withing a certain distance it drops several points. For example a single bullet could have a suppression value of three, which decreases by one for every meter between it's landing point and the infantryman in question (so past 3m it has no effect). A grenade explosion could have a value of 30, dropping quickly over range at a rate of 2 points per meter, so no effect past 15m. A frikkin' 500kg bomb would have a value of maybe 300 or more, but fall off faster at a rate of five points per meter (so no effect past 60m range).
The Courage Bar™ would have three thresholds, with each forcing the infantryman to drop down farther due to suppression.
Standing would require 80%+ courage.
At >80% the player is forced to Crouch.
At >50% they are forced Prone.
Below 20% they are completely Suppressed and curl up into a fetal position, unable to move to return fire.
So how does this place out? Just like in real life, or at least the movies. One bullet whizzing past might make you flinch, but a SMG spray nearby would definitely make you duck. A bomb going off, even on the other side of a wall, would make you hit the dirt. And having some psycho hosing down your position with machine gun fire is going to completely ruin your game plan no matter what. Of course, as first mentioned, the bar does recover over time (and fairly quickly) so it's not like it's a permanent condition.
Simple, neat, and effective. XOOM, print this out and magnet it to your refrigerator.
Note: This is also one of the few places where friendly fire might be allowed, at least for explosives such as bombs, mortars and (some day) artillery, in that while they might not kill friendlies (true FF) the courage loss could still happen (indirect FF).
"But if a 500kg bomb lands right on me I'd be forced into suppression for 5 minutes!" Buddy, if a 500kg bomb lands right on you suppression won't be an issue, trust me. Anything that could lock you down for any real length of time will have either already killed you or be just as locked down himself trying to keep his machineguns fed. Yeah, if you try to rush five tanks and they all open up at once you're probably going to be wetting your pants in that berm for a while - and you deserve no less. No, you can't just snipe that ATG after it dropped a 50mmHE round directly behind you even if the RNG gods spared you from an instant shrapnel death. Six guys chucking grenades into a bunker all at once will be an effective tactic even if they survive the blasts. An He111 dropping death nuggets from on high is cause for panic. A LMG actually would be a support weapon and not just a crutch for people with bad aim. So many improvements, one simple mechanic.