Hit points and damage are inextricably linked, and the abstractness of HP implies that the damage needs to be equally abstract to make any sense. It’s ludicrous to imagine a human warrior being impaled by multiple spearmen and fighting on, so those spearmen must not actually be stabbing the warrior when they keep rolling 6s for damage. That's why the don't-get-hit justification was needed in the first place.
Just one arrow and Conan is hurt. You are not tougher than Conan. Art by John Buscema
It makes sense in theory, but the way it plays out at my table, it’s pretty clear that damage specifically represents physical injuries. (“Shit, I rolled a 2. I just barely nicked him.”) Therefore, at my game table, hit points don’t actually represent anything other than physical endurance. How much pain can you take before you die or pass out? That’s what hit points are. Which means two things: hit points are way too high and always have been*, and I need something else to represent everything about them that isn’t physical endurance (the don’t-get-hit part of hit points).
One of the few ideas that’s remained consistent across every edition of D&D is the notion that armor keeps you from getting hit, but that doesn’t model reality particularly well either. What armor actually does is keep those hits from doing as much damage as they usually would.
Here, take a look at what bayonet training in the army looks like:
There's more to it than just the pugil sticks but this is the part that's relevant here.
In game terms, those guys are wearing padded armor. +1 AC. Maybe +2 depending on how you handle helmets. Does it look like the padding is making it easier to avoid attacks?
The idea that AC keeps your character from getting hit implies a couple things. One is that all armor, when struck, absorbs all force perfectly and completely. No armor made by human hands does that, but if you accept it anyway, the next logical conclusion is that a better AC just means that more of the body is covered. Which makes sense for the difference between a breastplate and full plate armor but that’s the exception.
So, to avoid having to say things like “The spear tip bounces harmlessly off your leather jacket,” AC means damage reduction at my table. Plate mail doesn’t keep you from getting hit with a hammer but it’s not gonna hurt as much.
Now that I’ve addressed what doesn’t keep you from getting hit, I can talk about what does. It’s exactly the same thing as what helps you hit your own target: training and experience.** Which, in D&D, mean class and level. There’s a number for this that’s been called attack bonus or base attack bonus depending on which edition you’re looking at. For me it’s just combat bonus.
Example: Grok the fighter is attacking Alita the thief. Based on class and level, Grok’s combat bonus is +4. He rolls a d20 and gets 13, 17 total. Alita doesn’t roll anything, just adds her combat bonus of +2 to 10, ahead of time, calling it her defense bonus. So Grok’s 17 beats Alita’s 12 and he rolls 1d8 damage, getting a 4. Alita is wearing padded armor (AC +1), so she takes 3 damage.
There are two things I’ve noticed about what this means in actual play. For one thing, armor isn’t as important as it used to be. It has an effect, but only if you get hit, which warriors are better at avoiding just because they’re warriors. My character sheets make encumbrance pretty easy to avoid in much the same way the LotFP sheets do, so I enforce it pretty strictly; this means that the pros and cons of wearing any armor at all are pretty situational and often end up being a wash. So Conan can run around in a loincloth and get along just fine. Which is awesome.
I might as well also mention that I nixed armor restrictions while I was at it, since armor doesn’t matter as much (especially since I use early firearm rules that ignore the armor entirely anyway). It hasn’t actually come up in play, possibly because mages and thieves with low strength are taking a bigger hit on encumbrance. I could see a mage using armor to become less a glass cannon and a little more of a tank, but that doesn’t feel like it breaks anything to me. Aesthetically speaking, I see magic users as less Gandalf and more Elric anyway.
So, to summarize the house rules I’ve discussed so far:
1. Class based damage, no weapon or armor restrictions.
2. AC = Damage Reduction.
3. Hitting and not getting hit are both based on Combat Bonus, which is based on class and level.
See? The rule changes aren’t complicated, just the reasoning. Bear in mind that I’m not arguing anywhere that these changes would improve your game, just that they do a better job of modeling reality. Many players and refs prefer to revel in the abstraction. My group has more fun with combat with the changes in place, measured by how into the narration we get and how much time we spend laughing maniacally.
*I re-wrote the class progressions, too; my version works as a single table that covers all the classes I use. I’ll type it up and post it at some point and you can see from that how I addressed absurdly high HP totals.
**There’s also a natural ability component, but my take on ability scores is probably a whole other post so let’s ignore them completely for now.