I was playing Left 4 Dead last night with some clanmates, and we were having a jolly old time goofing off and shooting each other in the back when there were zombies bearing down on us from all sides. The thing about L4D is that Survivors fall over when their health falls past a certain point; not dead, but just unable to move until a teammate saves them. (And in our friendly-fire match, this led to hilarious griefing sessions where we downed each other to be used as zombie bait.)
Then I read the past couple posts.
I realize now that in most games, developers are often shy about taking control away from players in some form or another. Blizzard withheld knockback in WoW until last year, and there has been considerable outcry against Valve for the recently introduced stun weapon in Team Fortress 2. Similarly, grenades in FPS games have always done "about as much damage as a rabbit fart", when realistically a small detonation would tear someone up from even ten feet away.
In Left 4 Dead, gameplay centers around incapacitating Survivor players; the Special Infected "boss" zombies aren't meant to deal inordinate amounts of damage, and in fact are usually no more durable than the regular zombie, even as playable characters in Versus mode. The real power of the Special Infected comes from their ability to change the movement of the Survivors; they can stop their movement, separate them from the group, and even push or pull them into hazards.
Do the Survivor players complain? Never. For them, getting downed, stunned, knocked around or dragged halfway across the map is a fact of life, an accepted part of the kind of experience L4D aims to provide. Their first thought isn't "OMG STUNLOCK I RAGEQUIT" but rather "Oh snap! I hope my teammates come save me."
An aside: explosives don't send any characters flying, save for Special Infected corpses (which go flying half a mile anyways, even if you killed them with a teeny tiny pistol).
I think the "modern FPS gameplay of robotic GI Joes that run zigzag circles at full speed" isn't just an AI issue, it's a systemic problem that can be traced back to the fact that the modern FPS genre is founded upon the "lone Rambo juggernaut" narrative, which has threadbare room for real player vulnerability. Physics-based gameplay mechanics add a whole new layer of vulnerability and interaction outside of the "point and click to change this number representing health" scheme.
In the end, what we end up with are a bunch of characters that run around shooting guns at each other and not much else; you could do a "demake" of most modern FPS games into something resembling Wolfenstein 3D and the experience would not change much.