Thought Experiment: Everything as HP

EDIT: I made some clarifications and modifications. 

INTRODUCTION

To understand why I want to propose what I'm proposing, I'd like to first consider the different views/definition in the earlier edition of D&D given for HP (Hit Point, or sometimes translated in other games as Health Point). As an aside, I suggest anyone to read this essay by the Alexandrian for the conceptual portion and this one by The Rot Grub on the history/stats aspect of D&D and finally this other excellent essay on HP by EmperorPonders.

Chainmail
There was no "hit points" in Chainmail per se, but there was "hits" considered under wargaming conventions. For example, some figures, like the Rocs, required "cumulative hits equal to a number sufficient to kill Heavy Horse to be killed themselves." Sometimes, it was wholly dependant on context, such as with Ogres:

Ogres are killed when they have taken an accumulation of six missile or melee hits in normal combat. Elves can kill them with three hits, and Hero-types or magical weapons kill them with a single hit.

A "hit" meant a kill for normal figures against normal figures. Therefore, the Chainmail "hits" are more akin to the "hit dice" as we will see with regards to description. This is even more evident considering that a Hero-figure, which required four hits to be killed, is now equivalent to four men in OD&D combat (i.e. 4 hits).

OD&D
Since Hit Points (each hits now dealing 1-6 hit points and hit dice providing 1-6 hit points) were a kind of oddities in the OD&D rules when you look at it from the historical perspective of wargaming, definitions aren't clear cut.

In Men & Magic, you have, regarding Hit Dice in the Fighting-Man description:

(the score of which determines how many points of damage can be taken before a character is killed)

And in the section Dice for Accumulative Hits (Hit Dice):

This indicates the number of dice which are rolled in order to determine how many hit points a character can take. Plusses are merely the number of pips to add to the total of all dice rolled not to each die. Thus a Super Hero gets 8 dice + 2; they are rolled and score 1, 2, 2, 3, 3, 4, 5, 6/totals 26 + 2 = 28, 28 being the number of points of damage the character could sustain before death. Whether sustaining accumulative hits will otherwise affect a character is left to the discretion of the referee.

 I do agree with the overall sentiment of The Rot Grub when he says:

Thus, saying that a character in OD&D had “two hit dice” was shorthand for saying they could approximately withstand “two hits.” Gygax’s original idea of hit points as abstract — encompassing the ability of high-level characters to dodge blows — is seen here quite literally.

In Greyhawk, we have:

hit points which can be sustained.

BECMI
In the Red Book, we find this:

Damage and hit pointsIn the game, when any creature is hit (either monster or character), damage is caused. There is a way of keeping track of damage, called hit points. The number of hit points is the amount of damage that a creature can take before being killed. Hit points can be any number; the more hit points a creature has, the harder it is to kill. We often use an abbreviation for hit points: it is hp. Your fighter starts with 8 hp (hit points) and still has all 8, since the goblin never hit you. He may have hit your armor or shield, but never got through your protection, so these attacks are still called “misses” - they didn’t actually damage your character.  

Constitution: Your health Your fighter is healthy, and can fight a long time without tiring. This ability is measured by another Ability Score, called Constitution. Your Constitution is 16, well above average but not perfect. Your Constitution affects your hit points. If you have a low score, you might only have 2 or 3 hit points. On the other hand, if you had an 18 Constitution, you might have as many as 10 hp, or more!

Up until now, it is still pretty evident: HP is about damage. Yes it's an abstraction, but it's clearly a mark of damage. They are caused by hits ("the goblin never hit you").

Also, an important note:

When the snake’s hit points become zero, the snake is dead. (If your hit points ever reach zero, you’re dead!)

AD&D 1st

This is where things get "bigger". In the PHB, you have (all underligned are my own):

Each character has a varying number of hit points,' just as monsters do. These hit points represent how much damage (actual or potential) the character can withstand before being killed. A certain amount of these hit points represent the actual physical punishment which can be sustained. The remainder, a significant portion of hit points at higher levels, stands for skill, luck, and/or magical factors. A typical man-at-arms can take about 5 hit points of damage before being Killed. Let us suppose that a 10th level fighter has 55 hit points, plus a bonus of 30 hit points for his constitution, for a total of 85 hit points. This is the equivalent of about 18 hit dice for creatures, about what it would take to kill four huge warhorses. It is ridiculous to assume that even a fantastic flghter can take that much punishment. The some holds true to a lesser extent for clerics, thieves, and the other classes. Thus, the majority of hit points are symbolic of combat skill, luck (bestowed by supernatural powers), and magical forces.

In the DMG, you have (all underligned are my own):

It is quite unreasonable to assume that as a character gains levels of ability in his or her class that a corresponding gain in actual ability to sustain physical damage takes place. It is preposterous to state such an assumption, for if we are to assume that a man is killed by a sword thrust which does 4 hit points of damage, we must similarly assume that a hero could, on the average, withstand five such thrusts before being slain! Why then the increase in hit points? Because these reflect both the actual physical ability of the character to withstand damage — as indicated by constitution bonuses — and a
commensurate increase in such areas as skill in combat and similar life-or-death situations, the “sixth sense” which warns the individual of some otherwise unforeseen events, sheer luck, and the fantastic provisions of magical protections and/or divine protection. Therefore, constitution affects both actual ability to withstand physical punishment hit points (physique) and the immeasurable areas which involve the sixth sense and luck (fitness).
Harkening back to the example of Rasputin, it would be safe to assume that he could withstand physical damage sufficient to have killed any four normal men, i.e. more than 14 hit points. Therefore, let us assume that a character with an 18 constitution will eventually be able to withstand no less than 15 hit points of actual physical damage before being slain, and that perhaps as many as 23 hit points could constitute the physical makeup of a character. The balance of accrued hit points are those which fall into the non-physical areas already detailed. Furthermore, these actual physical hit points would be spread across a large number of levels, starting from a base score of from an
average of 3 to 4, going up to 6 to 8 at 2nd level, 9 to 11 at 3rd, 12 to 14 at 4th, 15 to 17 at 5th, 18 to 20 at 6th, and 21 to 23 at 7th level. Note that the above assumes the character is a fighter with an average of 3 hit points per die going to physical ability to withstand punishment and only 1 point of constitution bonus being likewise assigned. Beyond the basic physical damage sustained, hits scored upon a character do not actually do such an amount of physical damage.
Consider a character who is a 10th level fighter with an 18 constitution. This character would have an average of 5½ hit points per die, plus a constitution bonus of 4 hit points, per level, or 95 hit points! Each hit scored upon the character does only a small amount of actual physical harm — the sword thrust that would have run a 1st level fighter through the heart merely grazes the character due to the fighter’s exceptional skill, luck, and sixth sense ability which caused movement to avoid the attack at just the right moment. However, having sustained 40 or 50 hit points of damage, our lordly fighter will be covered with a number of nicks, scratches, cuts and bruises. It will require a long period of rest and recuperation to regain the physical and metaphysical peak of 95 hit points.

Other Views
In Gary Gygax's Master of the Game, HP is considered as "disablement factors":

This table subsumes that the target will have "disablement factors" (in some systems called "structural damage capacity" or "hit points") between 2 and 12, with the game personas having between 7 and 12 ( d6 + 6) to give them an advantage on the average.

Sub-conclusion
It seems clear from all of this then that HP were, at that point in time, understood as an abstraction that needed a rationale. The rationale is thus: hit points are not only an acuality, but also a potentiality. 

Picture by Jeff Easley

EVERYTHING AS HP

Therefore, I'll use the definition of EmperorPonders for HP:

Hit points loss is not an objective scale of physical trauma; it is relative to the level, equipment, and many other conditions of the target who suffers the damage

Therefore, what if we used that particular abstraction that are HP, which comprises many aspect of the characters, to also track other things? What if HP was the new metric?

So if we take all the definitions of HP presented above, we can say that HP represents:

• Physical fitness
• Combat skill, prowess and capacity
• Luck and blessing
• Magical protection and modifiers

This means that, on the contrary, what "reduce" the abstracted attribute of the character named Hit Point are the opposite, meaning:

• Injury and illness
• Anything that contribute to lowering the effectiveness of a fighter's skill, prowess and capacity in combat
• Curses and badluck
• Magical impairment

Let's take each of those at a time to construct our HP-as-everything system.

First, we need to define what system of HP we need to use for this. Since I like it both simple and compatible, this would be the basic rules:

• use whatever HP/HD you have in your current edition, BUT at character creation, everyone receive an extra dice per their class
• when you are hit, you receive damage as per normal
• you have a static maximum HP, which is your overall HP, and a current max HP, called MHP from now on
• if you are below 0 MHP, you either die (in combat or because of a trap) or fall unconscious (outside combat)

Injuries & Illness
If you fall below 0 HP, instead of dying, you lose 1 MHP per damage point received. Those are injuries. It is up to the Referee to determine what kind or roll on one of the hundreds of "Death & Dismemberment" rules that exist. If you do this, just abstract whatever negative modifier by the rule above of losing maximum HP.

If you develop an illness, you lose -25% or -50% MHP as per the severity of the illness as long as you have the illness.

Even bad weather could lower your MHP (just not below 0).

Combat Effectiveness Modifier (Encumbrance)
We could name quite a few things that can lower combat effectiveness, but one that's easy to determine and abstract for a multitude of variable would be encumbrance. If you are in full plate with a huge-ass bag full of gold, you slow down your movement rate, but also your overall capacity. Therefore behold, "encumbrance as HP":

• Worn Armour:
• Shields are -1 MHP
• Medium Armour are -2 MHP (also slow down movement rate and make swimming difficult)
• Heavy Armour are -4 MHP (also slow down movement rate and make swimming impossible and climbing difficult)
• Hand-held weapon have no modifier
• Item that are on the belt or as bandolier have no effect. Rather it's up to the Referee to judge if it's "too much", but the player should be honest about it. 
• Insignificant items (rings, small gems, etc.) are worn on the belt
• Transported extra weapon are -1 MHP
• Each two items of significance are -1 MHP
• Quivers and such that transport stack of an item are -1 MHP for 10
• 250 GP is -1 MHP
• If you are paralyzed, your AC doesn't change but your MHP is down to 0 (meaning that anybody can kill you if they take the time to do it)
• You can never volontarily go beyond 0HP by wearing equipment. If you are at 0 HP, all future hits will subtract your MHP and you will die. 

This also have a lot of positive side effect. You want to play a barbarian without armour? Well you have more mobility and reflexes (i.e., more HP). The thief is in light armour and should be good against traps? Well now he has about as much HP as the fighter since he's not wearing armour. Etc. With this, HP becomes a mean of determining overall reflex and mobility. 

It also means that a high-level fighter is more "suited" to figthing in armour and has a better overall endurance; whereas a commoner in armour will be protected, but will lose all mobility and overall fitness/endurance which means he will die quick if hit. 

Curse & Level-Drain
When you get a curse, no matter the effect (disfiguration, strange skin color, etc.) you lose 1 MHP. When you get level-drained, instead of losing level, you lose 1d6 MHP.

Recovering MHP 
MHP lost temporarly (for example, encumbrance), is regained when the action is done. Others, such as level-drain, needs to be recovered as per normal (remove curses, restoration, etc.).

Example

Marcus the level 1 3LBB fighting-man. Let's say that he rolls 3 and 5, with a Constitution bonus, so a total of 10 HP. This means that he has:

• A Real Maximum Hit Point total of 10
• A MHP of 10
• A current HP of 10

He decides to carry the following:

• Sword (0)
• Shield (-1)
• Chain mail (-2)
• Rope as bandolier (0)
• An extra knife on his belt (0)
• A bedroll and a waterskin (-1)
• Two rations (-1)
• Two torches (-1)
• A small hammer with 10 iron spikes (-1)

So he has:

• RMHP: 10
• MHP: 3
• HP: 3

He fights a goblin, the goblin is lucky and scores a hit of 4 before dying. Now, Marcus has:

• RMHP: 10
• MHP: 2
• HP: 0

So if he receives another hit of 2 or more, he dies.

Of course, names are up to change, since it's unclear now what is what. Also, the system breaks down if you play an edition with d8 or worse, a d10, and the PC is level 10 with above 75 HP. My guess is either that in other edition, you have to double all encumbrance effect (effectively lowering the chance of lower-level HP), or you have to come up with something else. In OD&D I can see this working, especially considering that only fighters really reach above 20-30, unless you have a level 15 campaign or something, which I don't. 

CONCLUSION
This is just an experiment, armchair theory, but instead of having "equipment slots as everything" (such as spells, injury, and what not), or GP as everything (as a measure of encumbrance), why not use HP which is already a metric that you HAVE to track anyway.

Also, I made sure that this is perfectly compatible. You don't have to change anything to your setting or anything, you just remove some rules (for example, encumbrance) and add another layer on already existing rules (HP).