I've seen some servers implement this. It would be cool to mortally wound someone. Kind of a cool addition to the squirting blood when you are wounded down to 20 hp.
After being wounded to this extent you would be considered mortally wounded, and you would expire shortly, even if you are not attacked in the meantime. So if you are wounded down to 10 or 20 hp or something you start loosing hp at a rate of 1 hp every 2 seconds. That way you still have time to act, but you better make it quick.