A Swing at Armor Class

Armor Class option:

Melee AC = size modifier + parry modifier + dexterity bonus + armor bonus + shield bonus

This replaces the third edition version:

AC = 10 + dexterity modifier + armor bonus + shield bonus

Explanation time:

Size modifiers:
 - fine = 8
 - tiny = 7
 - small = 6
 - medium = 5
 - large = 4
 - huge = 3
 - gargantuan = 2
 - colossal = 1

(This means base AC for an unarmed human is 5, rather than 10)

Parry modifier: if you are holding a weapon, you get a free +5 bonus to AC, representing your ability to BLOCK with said weapon. You may increase your parry modifier over time if you are a fighting class (see the post following this one).

All the above is only for melee AC, since you are able to block a melee attack. Ranged weapons are unblockable (with some exceptions), but it is harder to hit someone in general. Therefore:

Ranged AC = size modifier x 2 + dexterity modifier + armor bonus + shield bonus

(this means base AC for a human is 10, as in the standard rules)

Unarmed people should be easier to hit with a sword than armed people.

Further Reading: Propositions for Circumstances Regarding the Contact Between an Attacker's Weapon and a Defender's Weapon, Shield or Armor, by Alexis at Tao of D&D