The major issue of expanding the scope of hp is when they are used to indicate an actual telling blow without regard to their expanded role: the poison dart that inflicts its payload if any damage is scored, for example. Hard to figure how the poison gets into someone's system when the dart was a clean miss that only shakes the target's morale.
This is not a problem of interpretation, it is one of the many problems, issues, paradoxes of combining physical damage in with all the other things that hit points are designed to represent (sorry to keep hammering this in all my posts - I'm sure you guys are sick of it by now). If your character takes hit point damage then the poison obviously does not work. If they take physical damage then their body has to fight the poison; best represented by a suitable "save" mechanic.
If you expand the range of what hp represent, you have to make sure that any consequences of hp loss make sense under the new definition. If the description for hp loss is in the hands of the player and can range from morale, exhaustion, bruise/pull, or actual telling blow then the effects of rider powers, extra defenses like DR, incorporeal, etc. need to be rethought.
Not only do you have to make sure that the consequences of hp loss make sense, you also have to (and in some respects more importantly) have to make sure the way how hit points are
restored makes sense - this is where every single edition of D&D in my opinion has got it wrong.
A thought exercise:
Imagine that you do separate physical damage from everything else hit points represents. Now imagine that my character loses hit points as the result of having a greataxe swung at him; what has happened?
Luck
He got lucky - something that you can't rely on every time and so the loss of hit points makes sense. In terms of restoration - who can say how luck works? As such it is at the will of the narrative and most particularly how the dice roll in the first place so I'm not thinking there is an issue here with hit points as luck. Interestingly, I could see several feats or features growing out of luck impacting hit points with this interpretation. A halfling rogue just seems to get lucky more often (and thus may have more hit points because of it).
The ability to turn a serious blow into a less serious one
This would seem to encompass skill and combat experience and thus this element of hit points represents keeping one's head in battle. I suppose it is as much the mental side of things in battle. It makes sense that an experienced character would have more hit points than a novice because of this. It makes sense that as a combat goes on, a combatants capacity to maintain their highly-skilled abilities will be tested. A quick rest and the adrenaline is back on tap for the next encounter.
The ability to take physical punishment but keep going
This is something that reflects the endurance and toughness of a character. What would have some character's screaming like a girl is just an inconsequential flesh wound for others that will need to be stitched when opportunity presents itself sometime later. Where as the previous interpretation would seem to reflect willpower, this one seems to reflect fortitude. More hit points represents a tougher character well but in terms of interpretation, the complexity is now starting to add up. You might have a lucky skillful rogue and a tough as guts barbarian with equal hit points. When the rogue takes hit point damage, it is their skill and luck that is helping them avoid the worst of the blows while for the barbarian it is as much toughness and a seemingly infinite well of endurance as raw skill that keeps them going. The DM/GM is given a measure of scope here in describing the resolution of actions - and in fact, how a character gets their hit points neatly informs them how to explain the combat narrative to their players. And so this burgeoning complexity of interpretation seems to be manageable. As for getting one's toughness back, again I know of no metric that can measure this and so the restoration of hit points due to this will make sense. A rest, a kiss on the bruise and we're back into the action.
Divine Power
The 3e example of a paladin staring down the wizard as the paladin is engulfed in arcane flames, only to remain unsinged is perhaps divine influence represented most overtly. The higher the level of the paladin, the greater their relationship with their deity and so the more sense that hit points could be derived from such. Now their deity is not going to protect them all the time but here and there, the divine character can be safe in the knowledge that their god will keep them safe. The restoration of such divine providence again is completely unknown, and so dropping it into the hit point pot would not seem to cause an issue.
Morale
This is the will to keep going, even if it means a character sacrificing themselves for something they believe in (be that belief foolish or not). In some ways this ties in with the 2nd and 3rd points underlining them and so as such, they form this neat triumvirate of capabilities that all characters will share. Again, there is an expression of desire here which may or may not be appropriate to the character but, a higher level character's morale is not going to be as sorely tested as a novice because they would have more confidence in their skills versus the same opponent than the novice - as such more hit points makes sense. The restoration of morale is interesting though. Perhaps it helps define what a short rest entails. It is a chance to deal with one's dire situation as much as it is to catch one's breath. A character who is under the extreme influence of fear may not get as many hit points back as normal from a short rest.
Inner Power
Is perhaps connected to morale but would seem to be the province of certain classes such as the monk. That such inner power may be as quickly restored as luck, divine favor or anything else so mentioned would seem to mean that again, this would be a suitable addition into the hit point melting pot.
And so, my point is that all of these things seem to make a sort of sense in the fantasy world without breaking the internal logic that is accepted when entering that fantasy world. It is only physical damage of which our ideas are so firmly rooted in our own real world that causes the issues of logic. The disbelief that a character could be at death's door one day and then barely 24 hours later is at full and maximum capacity.
In terms of other defenses such as DR, incorporeality, regeneration and so on, I'm not too sure that there is a problem - if physical damage is taken separately, then everything seems to easily follow what is logical in my opinion. DR becomes something that is applicable to both hit point loss as well as physical damage as it is a protection; a measure of the threat in the first place. Incorporeality is interesting as their is no body to do physical damage upon. There would seem to be a reliance on positive or radiant energy to combat such creatures which would make them quite deadly. I suppose the usual process of magical weaponry being partially effective is reasonable too. Regeneration only works on physical damage, not hit point damage. If anything, I think this provides a definite clarity to the equation which has strained interpretations of previous versions of D&D.
Perhaps though where things get interesting and that hit points and physical damage cannot immediately cover is:
- Consciousness and getting dazed, stunned or knocked out.
- Fatigue and Exhaustion (although one could be 0 hps equals the fatigued condition which can then get worse, exhausted, then debilitated, then incapacitated).
- Fear effects (again, this is close with fear effects doing hit point damage that is not reduced by DR as much as by will defense).
- Drowning effects: linked to Consciousness I suppose but would seem to be independent of hit points and physical damage).
Anyway, thank you Nagol for the thought-provoking post - it provoked a lot of thoughts from me.
Best Regards
Herremann the Wise