As a DM I wouldn't want to do this - too much bookkeeping, given as I already have to track the opposition's h.p. as well.
As a player...it'd be an interesting experiment. Puts a lot on the DM's plate to make sure our conditions get described in a way we can relate to.
Hit points to us are a vague combination of fatigue, physical damage, and stamina; with every hit to some extent affecting all three mostly so poisoned weapons can work without too much head-twisting. Thus, a character is usually going to have a pretty good idea what shape it's in. We also use a variant on a wound/vitality system (we call it body/fatigue) and you'll certainly know when you're into body points - and as most people only have a few of these, getting into bodies means it's probably time to either bail out or prepare the blaze of glory you're about to go down in.
When I describe monster hit points, what I say depends somewhat on what they're fighting. Something like an ogre - yeah, you can get a pretty good idea how it's doing. But a wraith? Not a chance, though you can usually tell you've affected it you've no real way of knowing what it's still got in the tank. But yeah, for most foes once they get it down to half or one-third I'll let them know it's looking somewhat the worse for wear, and once into bodies I'll tell 'em it's bleeding and wobbling.
pukunui said:
Likewise, how is a healer going to know what level spell slot to expend on a healing spell?
How about a perception roll or similar to narrow the recipient's condition down a bit? Answers in different situations might range from "You can tell she's almost as healthy as she can get" to "You're sure that no matter what variant of cure you use, one won't be enough" to "You think a minor cure ought to do the trick" to "You have no idea, other than she's at less than her usual fighting form". Another factor would be length of association - knowing the capabilities of someone you've run with for ten adventures is a lot more a sure thing than of someone you just met.
Lanefan