I look at combat in AD&D now, at the level ~5 range and...I run into the problem I've talked about before. You cannot include a statistical chance of death, without instigating a certain death sentence. But without a chance of death, the thrill that you get at level one ("OMG am I gunna die!!?!?!") is absent.
I'm considering this idea now were at higher levels: Saying that various monsters attacks are so vicious that for some of them, if they drop you to the negatives you suffer a long term loss in the maximum number of hit points you have. When you go down, you make a system shock save. If you pass, you lose 5 hitpoints off your max, for this session and one future game session. If you fail, it's for this session and two future game sessions. You can gain this penalty twice and the HP penalty stacks, but no more than that.
The reason I say game sessions instead of just weeks is because if at the start of the session the players just go 'oh, we sit around in the tavern for two weeks - there, effects gone! Then we adventure!' it sucks. It's the most weak ass way of avoiding consequences - it's just a free get out of jail card. That sort of stuff is for pure simulationists who don't care if the game is challenging for players (they only care about playing out a world, no matter how easy or hard that makes gameplay).
So the idea is that it will definately affect future sessions. It may even bring about your demise! But since it doesn't kill you in itself, it gives some wiggle room for the players to avoid death (so it's not just a statistical implementation of a death chance).
Possibly at levels 10 to 15 it might become a 10 hit point penalty. 16 to 20, a 20 hitpoint penalty.