A lot of game Systems use HP. Few, strangely, give Tiers of Conditions or Modifiers to the condition of the character from the extend of the damage (ex. Fading Suns and L5R and Traveller to name a few; This kind of system is called a "Death Spiral"). Fuzion and Hero are HP system with some quirks like the Stunning and lethal damage.
Wound Threshold or Wound Save system is simply a check/roll against Damage (more damage the lower chance of success) modified by things that define "toughness". WOD, Savage Worlds, and some D20 variants use these rules - I happen to think they, Wound Save and Thresholds, are the most elegant wounding rules because it can be translated so easily to many systems and allow you multiple assumptions where you just adjust benchmarks back and forth.
GURPS is a hybrid of Wound Threshold/Save system and of HP system. You check against the amount of damage (in essence you have a damage threshold) until you reach an amount of HP damage that physically (the system assumes in certain conditions) is Impossible to survive from (in GURPS its HPx6 to HPx11).
WH40k is a bit of a mixed HP system - many other stuff happens when you get damaged. Its more fun when you automate the statistics instead of checking on the book (memorizing it is not an option if this is not your system of mastery). You can generalize that Systems with Tables for Damage and all the conditions it will result is a Mixed System for HP. There are conditions but you have to consult a table which is tedious and best automated in an app the GM can just click IMO. To stress this more, try to time your game referencing and look at the Opportunity cost of checking such references to Playing. This method of analysis works in other aspects of games if you can record and measure them, the time you spend on something and if there are better uses of the time that will bring out more fun.
In GURPS marital arts there is a more lethal "death spiral". Although I don't use them because, I find just enforcing the half dodge and movement rule already lethal (all your enemies are more likely to do Telegraphic Attacks) when you apply simple tactics and I find it additional book keeping. I learned in airsoft, when you've hit the wall everything goes down hill.
Its pretty easy to port the Wound Threshold system and the Death Spiral into many systems if you don't like the current system. You can port GURPS hybrid threshold HP system to 3.0-3.5 DnD (shock penalties, Fort saves when taking large amounts of damage, and a bit of a death spiral). In fact you can take a mechanic from any game system and translate it to another if you can understand the basic statistics of their success rolls (which is why I have a hard time with systems that use a lot of dice).
Wound Threshold System
Simply Take the Damage and use it against what counts as Toughness or Resistance to Damage, what is at stake is the Condition of the person or object taking the damage. The Damage can be static and the Defender roll their Toughness/Resistance or vice versa. Note that the more only one side needs to roll (player vs GM) the faster things go.
Damage can be a Margin of Success of a Hit, to show their correlation. Armor affects Toughness and so are other intrinsic conditions and qualities. You can have classes to represent various rock-paper-scissors situation in wounding (Rock beats Scissors etc...).
In GURPS I made damage as a Task Difficulty Modifier, affecting the Defender's Toughness roll. In D20 it can be modified Fort Save vs a Static Damage number. Note that toughness being the variable can replace variable damage, Margin of Failure is important if conditions have multiple tiers. So a very bad failure can bring a character down multiple tiers of conditions.
Ideally the players need to bring up their Attack to inflict more damage or add more conditions that will make it difficult to save against the Damage if they are gaming the system. So what was typically adding more damage, becomes increasing the damage options.
Hit location like legs, hands, feet etc... can create damage conditions. These conditions modify the Wounding System with lowering the chance of Saving against the damage but limiting the conditions from Death to incapacitating the Defender.
I just learned while writing this creating a blank Rules Template for this would help... but given how niche this is I'm better off making more token drawings.
Damage Class and Wound Systems. GURPS has a very simple damage class system - which is scaling damage in 10s, and hundreds. Wound Threshold system can also have a damage class, a modifier to a "class of attacks" that are really far greater or for weaker. Small creatures can have a Damage class, a modifier, to all its attacks making it weaker against larger opponents and you can have a damage class against Fire Arms or Weapon batteries!
You can create a damage class system based on any arbitrary value, technological leaps in protection and damage (like in War Hammer and Rifts), and simply looking at all the damage scales in your game system and grouping them to simplify.
Note that the Wound Threshold System, Death Spiral, and Damage Class or Scale can be applied to in animate objects.
So the search for a better model, and a model that is easier to implement (less points of potential failure or disaster), goes on. This compulsion only applies to some, and those so thoroughly afflicted create heretical constructs derived from many systems "hacked" together to create a monster. Their games run smoother, with a dangerous edge while the GM can pull levers and press buttons that create special effects that dazzle and maintain the powerful illusion of the game and its world.
Disclaimer: Experimentation leads to Hersey, Doubt, falling outside the Norm, and a disruptive behavior. For your social comfort and stability do not experiment without the supervision or permission of an Authorized and Certified GM. You will comply.
Edit: Notes from Comments:
- Dice Pool or Value Pool System - in Tunnels and Trolls combat performance is by rolling a pool of dice that is then distributed - allocated against an opponent or opponents. This creates wound conditions for those who "lose" against the higher value. I forgot the system, but I played one with Dice Pool that shrinks as one takes wounds - I think its Aberrant or some WoD system.
- Not all HP systems are the Same - Some HP systems are very limited. Fading Suns and Qin The Warring States have a HP system that averages around 10 give or take 50% to double (and sometimes more). Some are a lot of points like in DnD. Some is Mixed like in the early Star Wars D20 with "Vitality" representing Gygaxian HP and Con being the "physical" HP.
- Soaking Systems. Some Alternate to HP with a few Tiers of Wounded conditions use Soaking Systems like WoD where Damage is "Soaked" up by Toughness or The Endurance Stat equivalent. Failing to Soak is damage and you track it like HP. Some systems are more lethal, others can be dialed up or down in lethality. Soaking Systems are not so different from Any System that lowers the Damage or makes damage the equivalent of Checks creating a Binary Yes or No if the character is wounded or Not.
- Narrative Control. House of the Blooded is the only system I got to play that uses Narrative control for damage. if you beat the guy (there is a wager system, a kind of dice pool penalized as you want more and more narrative control, more opportunities to declare facts) you declare what you do to the opponent. You can declare one "fact" like Killing him, but if he also makes a mistake, he can also say the other guy Dies of his wounds. To say I killed him and Survived is two facts, while the opponent can also say he survived (restating a fact does not cost any "facts), but is imprisoned for the murder. etc...
Edit: the Few Systems I know (will update)
- Qin TWS - HP with Death Spiral
- L5R - HP with Death Spiral
- GURPS - HP with Thresholds, and various penalties. Optional rules for gritter and more dangerous Death Spiral.
- Hero - HP with non-lethal damage and Thresholds.
- Tunnels and Trolls. Damage is from values rolled by a Dice Pool, reallocation of the value per opponent afflicts Wounding Conditions
- War Hammer 40k - HP system, with Critical hit Tables. These tables act like variable conditions.
- DnD 1.0-5. - HP system. In Later version with various damage conditions
- Harn 3.0
- WoD - Mostly the Core system uses Damage Soak, with a Few Wound Tiers (which can be viewed as HP)
- Traveller Classic to Mongoose - Stats as HP, Death Spiral from Stat Loss.
- Fading Suns 1-3.0 HP system with few points, with death spiral.
- D20 Alternate Rules - Wound Threshold with improving Toughness Saves.
- House of the Blooded - Narrative Control
- Mech Warrior / Battle Tech - (will review)
- Rifts (and Many Palladium Games) - HP system
- Dungeon World -
- Dead Lands - Wound Saves/Threshold.