Just read this twist on the reaper threshold concept over at Jeff's Gameblog. It's from the original Chainmail rules, but its the same premise: just cuz you're out of hit points doesn't mean you're dead. This is a much more qualitative approach and I think I actually prefer it. Since I was applying my idea to Castles & Crusades rules, I might add the generic Constitution bonus to the roll instead of adding +1 for 1st level characters... except that that would make someone with an 18 constitution very hard to kill; you'd have to roll a 1 and then fail 3 saves vs. Death Ray.
Which raises another point: how come I haven't seen any Death Rays in the Old School gaming movement? C'mon guys!