So Jason (the player behind Cain) has always thought that Hit Points as they're currently represented are highly unrealistic. I think that they work fine for short-term combat and fatigue representation, but there is a problem that he pointed out: there's no long-term penalty for not recovering hp. If you're really that worn down that you can be finished off by one more sword-stroke (in the case I always present it as, your defenses are so decreased by fatigue and the buildup of small damage that the next blow is the one you can't properly defend against, sending you to the grave), then there should be some other effects as well.
Dungeoneering is tough. Getting hurt is tough. Having low hp is a pretty big penalty. But is it penalty enough? Last night he said to me: "Ahh, but you like a really realistic D&D. And not having long-term combat damage effects isn't really realistic."
SO HERE IT IS:
Players should record two new numbers -- 1/4th and 1/2 their PCs max hp. Once a combat ends, if they are below either threshold they sustain a (cumulative) -1 penalty to all of their rolls except for damage. This includes saving throws and to-hit rolls, as well as surprise checks, etc. For every two days spent beneath 1/2 hp, this penalty accrues another -1. Half of the penalties are obviated when the PC passes the 1/4th threshold and the rest when they pass the 1/2 threshold again.
I'm gonna throw this rule into the grinder and see how she works.