Hit Point Scaling
[I should probably heed Joshua Dyal's words, but I can't resist discussing Hit Points.]
The advantage of ablative hp is that it allows for a more gradual, more manageable reduction in strength. Lots of relatively small hits give less variability in toto than a few relatively severe hits.
While that's certainly true, if our goal is to reduce variability, why are we rolling dice? And why are we rolling BIG dice -- d20 to hit, and often d8, d10, or d12 for damage?
Further, if hitting isn't necessarily hitting, why are we rolling for both to-hit and damage? Why don't we just have an Attack Point value we subtract from the other guy's Hit Point value each round?
Obviously we don't want an accounting war, and we do want some variability. There's a continuum here.
If we look at typical monsters along the power spectrum, we see that the damage they deal out increases to match the high Hit Points of high-level characters. This should surprise no one. CR-1/4 Goblins deal out 1d8-1; CR-1 Gnolls deal out 1d8+2; CR-2 Ogres deal out 2d6+7; CR-7 Hill Giants deal out 2d6+10; CR-10 Fire Giants deal out 2d8+15.
At every level, the opposition does enough damage to hurt the heroes and kill them in, say, three or four blows -- whether those are 7-point blows against a low-level Fighter or 24-point blows against a 10th-level Fighter.
Once we accept that everyone likes combat that takes three or four blows to finish someone, we can scale attack bonuses and damage versus defense bonuses (AC) and Hit Points a few different ways.
Right now, a 10th-level Fighter (79 hp) facing a Fire Giant can take about four shots (averaging 24 hp). Facing another 10th-level Fighter armed with a mundane bastard sword, he can take eight blows (1d10+5, averaging 10.5); versus a +1 sword he can take seven (1d10+6, averaging 11.5). Against a horde of 1st-level Warriors with spears, he can take countless hits -- 18 supposedly good shots (averaging 4.5).
If we halved the Fighter's Hit Points, he'd still be able to take two good shots from a 12-foot giant wielding a 12-foot sword -- or four good sword strokes from his equal, or nine spear thrusts from spear-carriers (who have a 1-in-20 chance of even hitting him). If we then make him twice as hard to hit, he's just as powerful as before, without all the hand-waving of hits that aren't hits, non-injuries that need healing, etc.
That's hardly the "wait for the crit" scenario painted as the only alternative to high-hp heroes. The brave Fighter hopes to slay the Fire Giant unscathed, but if he takes a hit, it really hurts. (Sounds right.) If he pairs off against his equal, he expects to take three or four good sword strokes before it's over, and he'll be sorely wounded -- but those blows won't all land in the first or second round. And he's still able to take an ungodly amount of punishment from the spear-carriers -- expecting 180 of their attacks before nine of them land and actually bring him down.
Or we could give everyone one Hit Point (or a single d4 Hit Die) and just ramp up their AC. That seems to be the Straw Man everyone's knocking down.
The latter inflates the importance of getting a lucky critical, which ends the fight. This is not fun; in effect you're waiting for the first guy to roll a natural 20.
In a variant D&D with both a Base Attack Bonus and a Base Defense Bonus, two high-level Fighters facing off would hit each other as often as two low-level Fighters facing off. Waiting for a natural 20 is quite an overstatement. And having enough Hit Points to withstand four hits versus eight quasi-hits is hardly waiting for one lucky shot to end the fight.
Again, there's a continuum. We can have all ablative defenses (hp) and no other defense (AC), or we can have no real ablative defenses (1 hp, 1d4 hp) and high hit avoidance (AC bonus with level), or we can choose any point in between -- all regardless of how heroic (powerful) or gritty (lethal) we'd like the system.
The same effect has been observed in systems like GURPS, where fights between characters with extreme skill levels boil down to whoever gets the first crit in. It isn't fun in GURPS either.
In GURPS -- which, by the way, always seems to get trotted out as the only alternative to D&D, one we have to take
in toto -- a defender's defense roll is independent of the attacker's skill or attack roll -- unless the attacker scores a critical hit. Thus, "high-level" GURPS characters are unhittable conventionally -- they only fail their defense roll ~1% of the time -- and only fluke criticals land.
That's nothing like attack bonuses versus offsetting defense bonuses.