On Friday, I was thinking about hit points and combat in D&D (LotFP is my flavor of choice). It exists in this nebulous space where one d20 roll is not necessarily one swing of the sword, but could be a series of successful moves that wears down the opponent's stamina (thinking of HP as stamina has actually really helped me to conceptualize it, I think). In practice, though, it's often more dramatic and just plain easier to narrate the combat as though it were blow-by-blow. Sometimes, that makes it feel like lumbering battleships trading salvoes and doing unspecified "damage" until one finally sinks.
This is the one major disadvantage of the HP system, at least at my table. It abstracts combat to the point where hits have no concrete effect until 0 HP, and I have to do a little bit of on-the-spot adjudication every time a player wants to do something more specific than just "I attack".
What could a slightly more granular, HP-less D&D combat system look like? I've always liked the optional rules for dueling in A Red & Pleasant Land (p. 143). They're designed to add depth to one-on-one fights. Essentially, a dueler rolls on a short d6 injury table for every hit taken after reaching 0 HP. Once they roll the same injury twice, the dueler is either dead or unconscious (that's why the table is short).
I've used injury tables like this before. But what if we took HP out entirely and skipped right to the table? It could go something like this:
So, you'd roll to hit vs. AC as normal, but instead of rolling damage, you'd roll d10 to determine where you hit. Defenders would keep track of how many times that body part has been hit and apply the mechanical effects as they come up. If the attacker wants to hit a specific body part, the defender gets to add the indicated called shot bonus to AC.
Weapon damage would be replaced by how many "hits" one attack is worth. d4 and d6 weapons (daggers, shortswords, etc) would do one hit, d8 weapons (standard swords, etc) would do two hits, and d10 weapons (greataxes, etc) would do three. Critical hits would increase the number by one. Since any successful hit by a large weapon is likely to be a one-hit kill, there's a tradeoff: you can't apply any bonuses to hit, just the straight d20 roll. I'd also consider a Strength requirement to even wield a large weapon - either 13 or 16.
Without HP increases to make higher-level characters more survivable in combat, you'd need to increase base AC instead. Fighters and dwarves would go up 1 AC every two levels; clerics, elves, and thieves would go up every three; and magic-users and halflings every four.
This system would make combat short and brutal, which would work for some games but not others. It would require a bit of a different mindset than D&D combat as usual - mindless hack-and-slash without a plan is going to result in a lot of dead PCs. Also, you'd have to modify the table for non-humanoid creatures. Bespoke tables for each creature would help make fighting them feel unique, but it's a lot more work than just rolling up HP.