In a game I DM'd recently (AD&D 3E), I did away with AC and split defensive scores into two categories:
Evasion/Deflection: Affected the character's ability to avoid being hit altogether, opposed by adversary's attack roll. Heavier armors severely reduced this ability. Lightly armored characters we also able to react more quickly and roll with attacks, reducing the chance of critical hits.
Absorption/Reduction: Affected the character's ability to shrug off damage, opposed by adversary's damage roll. Heavier armors were more effective at reducing the damage taken.
There were som