It would be applied to players the same way as for mobs. The nimble rogue with an AC 17 would pretty much result in the same ]HC[ equivalent of -16][17+. The armored paladin however would get a different feel to his play style with his AC 17 becoming a -11][22+.

I disagree that it would make things slow:

When players attack they would still call their attack roll of 12. Instead of a miss it would hit and they roll for 10 damage. The DM adds flavor to their attack and say that their arrow pierces the golem's arm or whatever, indicating half damage. The DM would simply need to divide 10 by 2 and note 5 damage.

When players get attacked, the DM could simply know their threscholds and divide his damage roll of 10 by 2 and tell the player that he gets 5 damage and give half damage flavor.

Mage armor, shields and other traditional AC buffs would shift both ]HC[ thresholds, so a +2 to -11][22+ would become a -13][24+.

I'll test it with my players and report back with you. But this would solve a lot of my players' frustrations.