I've said before that I prefer descending AC, starting at AC 9 for no armor and going down to AC 2 for plate and shield. So I won't go over that again. But I thought I'd toss out some ideas on what to do with descending AC numbers, besides determine chance to hit.
Movement: Ignoring shields and encumbrance, round the AC of armored characters to the nearest power of 3 and add 3 to get the base move.
Thief Limitations: If using Dex bonuses for thief skills, only allow the bonus if Dex <= AC x 2.
Combat Evasion: Compare the lower of AC, Dex, or movement rate to the opponent's AC, Dex, or movement rate; the side with the higher score can withdraw out of reach before the other side can strike.
Cutting Wounds: If you are using 1d6 damage rolls and want to treat cuts as different from bruises (requiring 1 turn to bandage,) add opponent's AC to your damage roll; on a 10 or more, the opponent was cut instead of bruised.
Stunning Blows: On the other hand, unarmed attacks can be treated as being less likely to do real damage. Add opponent's AC to damage roll; damage (1-6) is real on a 10 or more, but on a 9 or less, it's treated as subduing damage.