On Critical Hits

As I was reading the commentary on a recent Grognardia post about ignored rules, I came across this observation by Matthew Johnson:

And then there are the rules that EVERYBODY used that are actually nowhere in the books. For instance, I was amazed to find when flipping through the DMG recently that a natural 20 officially has no special meaning (in the example of play section -- you know, the one with the calcified bone scroll tube in the pool -- one of the characters rolls a natural 20 and it's nothing but a regular hit.)

This is one of the things that I'd embraced more or less every time I have run D&D: a roll of 20 has some special effect, double damage dice or max damage or whatever. I've always been pretty even handed about it, applying it to PC as well as monster rolls, but really - 5% of hits being critical is actually fairly frequent, and the simple system in OD&D wasn't really developed for it. Some kind of critical hit rule was extremely common in actual play, probably because the system is otherwise pretty bland as-is.

Unfortunately critical hits are the opposite of what D&D combat is supposed to be modelling. Hit points are, from OD&D right through to 4th edition, representative of something other than the raw physical capacity to take damage. A single to-hit roll represents, not a single swing of the sword, but a number of feints, parries, thrusts, slashes and so on, and 4 points of damage out of 10 may not represent any physical injury at all but rather exhaustion or draining of the endurance, luck, etc of the character. (In terms of the underlying system this also calls into question both magical healing and variable weapon damage.) Saying that a great hit results in extra damage may be dramatic but it creates dissonance with the underlying combat engine.

The impulse for this is to create the really devastating death blows we read about in Conan stories or medieval romances, when a hero smashes an opponent in the face or rends them clean in half. (Le Morte d'Arthur and La Chanson de Roland are both full of people getting cut in half, sometimes to the point where it kills their horses too.) But I would argue that D&D already models this: when a character dies, it is narratively open how they actually meet their end. Just because you rolled exactly the 14 you needed to hit that orc doesn't mean you didn't hit the bastard right in his eye so the arrow went clean through his skull.

Still, the combat system leaves the impression of missing a certain something, and it makes sense to put it on that crucial roll of 20. But what is really missing from the D&D combat system? OD&D combat is lightning-fast and extremely deadly for most involved. What I am thinking goes like this: if a character hits and it's a 20, the target must roll to save versus paralysis (favoring Fighters) or lose their next turn. It's pretty simple, only adding 1 die roll, and to a saving throw chart. I think it does a good job of preserving that "nice hit" effect, in fact moreso as most combat you'll find in a good sword & sorcery yarn tends to have periods where one or the other combatant finds themselves incapacitated, but manages to come back.

Has anybody ever used a system like this? Can I (should I) simplify it any further, or spruce it up, or use it as-is?