Another way to do this, not that I'm going to, would be to to damage absorption for armor. So lightweight armor might absorb 25% of the damage of a hit, medium 50%, heavy 75%. Or whatever percentages make sense.
To ease calculations have a chart with damage ranges and the HP damage that would result. Along with this, reduce a PCs target AC significantly (by, sa -5) so more (but not all) hits do damage.
But the disadvantage of this of course is the monster side of things, as well as monks, barbarians and so on. What happens when the wizard with no armor gets attacked, and how is it affected by mage armor and shield?