The best d20-based swords-an'-armor combat system I've seen is from the old Game of Thrones d20.

Armor provides DR rather than AC - basically a straight-up conversion of D&D AC value to DR. AC comes from Dex bonus, from shields (which provide much more AC than in D&D... +6 for medium and +8 for large, IIRC), and from a Defense Bonus, which increases with level like BAB... half progression for the noncombatants, 3/4 for the combat classes, and full for the dodgy rogue and duelist types. And a 1d20 roll that replaces the 10 base AC in D&D (and allows eliminating 1/20 auto-miss/hit).

HP increase with level, but not nearly as quickly as in D&D, and there's a wound system where blows that do more damage than your wound threshold (half your Con, by default, IIRC) can cause a wound that causes you to bleed HP until you get medical attention (which takes time and doesn't always work) and may knock you unconscious.

The setting is very low-magic, and it uses 3.0 Power Attack without all the multipliers you can stack on in 3.5, so it's difficult to reliably do enough damage to hurt someone through good armor DR, especially with a one-handed weapon. And shields are effective enough that ditching it for a two-hander is not the no-brainer that it is in 3.x. And smashing through the shield rather than trying to go around it is frequently a sensible strategy, too.

The primary melee classes get class features that increase their armor DR, too, which reflects the ability of someone experienced with armor to use it to turn solid blows into harmless glances.

And the ability to coup-de-grace someone with a dagger is reflected simply by a rule that says that if you have someone at your mercy, you can kill them. No messing about with attack rolls and DR and damage and hit points... just, "I stick my dagger in his eye. He's dead."

It still keeps Dex caps from armor, though, which are a pretty dysfunctional mechanic in and of themselves. If you want to reflect armor slowing people down in their AC (and, as someone with a lot of armored combat experience, I'm not convinced that that's something that we should want - see "turning solid blows into harmless glances", above), implementing it as a simple penalty makes a lot more sense than doing it as a cap.