I remember an old Dragon magazine cartoon where they have this barbarian tied to a tree studded with arrows and a firing squad is shooting arrows at him. The guy in the background says something to the effect of, "Ok boys, keep shooting, he's got thirty HP left."
think it could get bogged down with all the "exception" rules, but I wonder how much of these situations can't be solved with situational rules?
For instance, Executions. If you are tied up, and about to be executed, it's not a coup de grace attack. The executioner simply kills you with his attack. No roll. You're dead. You ability to avoid this fate is tied to any immunities/protections, or what you should have done to avoid this point.
Falling from great heights: no cap on maximum damage. It's 1d6 per 10 feet. period. High PCs may survive a 50 foot fall, but not falling out of an airplane after joining the mile high club is 500d6, give or take, and the math with solve the problem. terminal velocity ain't worth modelling, if it means people fall from airplanes and live.
Lava should inflict way more damage, just by proximity. being with 5-10 feet of lava in the real world causes burns. So crank up the damage by proximity to 1d6 damage per 10 feet under 50 foot distance. Or whatever Wikipedia says the safe distance is.
Being surrounded by hostiles should fall under the overbearing rules from 2e, which enabled the angry mob of peasants to take down the 20th level PC.
Sure, this could turn into a long list of situational rules, but I don't see how they would drastically change D&D, other than correcting a few of the more extreme unrealisms that come up.