The hit point abstraction has significantly changed since OD&D. Back then, the game designers were unrestricted by 40 years of D&D history and could stay consistent with their own definition of hit points.
I haven’t managed to find a copy of the rules, but from what I gathered on the Internet, in OD&D, hit points are meat. A human takes one hit and he’s usually out. Large or mythical creatures can sustain multiple hits before they die. That’s where the word Hit Die comes from. A creature with 1 Hit Die can sustain 1 hit before he dies, a creature with 4 Hit Die can take 4 hits.
If hit points are meat, when humans get bonus hit die from levels, things get a little confusing. How can a human get hit 10 times by a sword and live through it? The answer is quite simple actually; D&D is a heroic fantasy RPG and not a real world physics simulation. But still, some future game designers that didn’t share Arneson’s vision of heroic fantasy heroes thought that it would be “more realistic” to have heroes that avoid blows instead of taking them. Is it really more realistic? Not really. Your heroes are dodging fireballs using ninja moves, they’re avoiding the damage of a 100’ fall, they somehow manage to swim in acid and still avoid the acid, they manage to dodge a sniper’s arrow, etc…
Except that with the “more modern” definition of hit points, things get even more confusing.
1) Why does your armor provide a bonus to your AC? This is really backwards. The only way you can rationalize this is by assuming things like “you would have been hit but because of your skill you managed to deflect the blow”.
2) It makes AC and hit points redundant. Your AC is your ability to avoid physical blows and your hit points is your ability to avoid physical blows. Which one is it? Why do you even have an attack roll if your ability to avoid physical blows is modeled by hit points.
3) Why do creatures have damage reduction or resistance? This is really backwards again.
4) Why do trolls have regeneration?
5) Why do attacks deal effects like poison and paralysis? Either you avoided the blow, either you don’t. Make up your mind.
6) Why don’t spells such as Sleep, Web or Finger of Death deal damage? Why can you avoid a sword blow but not avoid a spell?
7) If the amount of damage represents how hard it is to avoid a blow, why don’t you get a bonus to damage for favorable conditions such as flanking, backstabbing or sniping?
8) Why do you get bonus hit points for high constitution?
And the list goes on and on.
With Arneson’s vision of hit points, you only have one thing to deal with: you’re playing heroes and these heroes start as peasants and become creatures of legend that have the stamina of an elephant. Everything else makes perfect sense if you assume that hit points are meat. But if hit points are your ability to avoid blows, you have to rationalize everything on a case by case basis. You’re constantly juggling with the multiple elements included in the hit point abstraction to justify why it works. And sometimes, you can’t explain it at all. And there worst is that you don't even get the benefit of more realism. You just add confusion.
Wouldn’t it just be easier and more consistent to stick to hit points as meat?