One of the reasons people said certain editions had a sweet spot was because the hp at certain levels was too low (or high).
3E had the highest variance since it starts very low relative to damage (orc hits you, you die), then surges very high with good Con modifier/polymorph.
4E's too high for some people, though some of that also suggests that damage per hit should be lowered.
You also saw some odd interactions in older editions with attacks that dealt damage based on level/HD or even raw hp. Like fighting a dragon that rolled good hp in 1st ed was... a bit silly sometimes, since it could one shot the entire party, often dropping people on successful saves.
So, what's a good number of hits to drop a PC? Let's say we're looking at three types of enemies in an adventure.
Wimpy goblins with daggers
Sword-swinging skeleton
Tree-smashing ogre
If we're not talking crits, how many hits should it take to drop a PC from each?
Is it okay if the ogre drops someone in one swing? Is it okay if it _kills_ someone (from full hp) with a crit?
Is it okay if the goblins need _at least_ 4 average hits to drop a PC (assume you're fighting a dozen of them)?
Now let's reverse that. You've got 3 PCs:
An unarmed halfling wizard
An elf paladin with a longsword
A half-orc barbarian with a greataxe
(using some slightly non-core options just to have greater variance)
How many average _melee_ hits from each should it take to drop the ogre? The skeleton?
Should any have a chance to not kill one of the dozen goblins in one shot?
Answer that enough times with enough variation and you can establish a baseline of expectations for damage. And, for many folks, probably quickly find your paradoxes.
I think we can safely assume we're keeping rolled damage, for example - so if we want a PC able to deal between 1d4 and 1d12 (potentially plus something)... and vice versa on the monsters. Well, your minimums and your maximums get interesting.
Like if a PC tends to deal 1d4+4 to 1d12+4 at 1st level, and we don't want minimum damage to kill the skeleton, it needs more than 5 hp. If we want non-crits to take 2 hits to kill a skeleton, it needs at least 12 hp.
You can then apply the same thing to PCs. If we don't want PCs to get one-shotted, then they'll need more than 12hp even at 1st level.