A method my group is using
My group uses a completely different set of house rules for our game. We borrowed bits and pieces from different games to come up with something somewhat realistic and at the same time keep the heroic atmosphere. Here is an example of our Class Defense system.
Monks - Get Excellent progression
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20
Barbarians, Fighters, Paladins, Rangers and Rogues - Get Good progression (1,2,3,3,4,5,6,6,7,8,9,9,10,11,12,12,13,14,15,15)
Bards, Clerics, and Druids - Get Average progression (0,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10)
Sorcerers and Wizards - Get Poor progression (0,1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5)
We use wound points (equal to Consitution score)
Armor and Natural Armor provide Damage Reduction vs. Wound Point damage. Armor can be bypassed by Brilliant Energy weapons, and Natural Armor can be bypassed by Bane weapons for that creature type.
Defense represents a characters ability to dodge, roll out of the way, jump back, parry with a weapon, basically whatever it takes to avoid getting hit.
Defense = 10 + Dex + Class Bonus + Shield + size + misc.
When adding Dex to Defense, we treat it like the old Armor Class rules did. So if someone is wearing Full Plate with a Dexterity of 16, they only get a +1 to Dex.
Right now in my game our level 10 monk has a Defense of 30, but he has no Damage Reduction... so he is hard to hit.. but if he is hit it hurts. The Rogue 8/Ranger2 has a Defense of 25, and the level 10 Bard has a Defense of 21.
Your typical first level fighter will have a Defense of 12, while a level 20 fighter will have a Defense of 32+
Defense stacks with multiclassing characters...
so a Rogue 5/Sorcerer 3 would have a +5 Defense (+4 Rogue +1 Sorcerer)