Basically, each armor type has a soft cap (there are some more rules to it though) and once over that point you only get a percentage of the AC based on your class. Shield AC (and an actual shield item not just an item with stats that can go in off hand) skips the soft cap stuff so it goes over the cap and you get the full AC.
And here are the defaults (which are based on pre-softcap overhaul from SoF I think...):
Code:
RULE_INT ( Combat, ClothACSoftcap, 75)
RULE_INT ( Combat, LeatherACSoftcap, 100)
RULE_INT ( Combat, MonkACSoftcap, 120)
RULE_INT ( Combat, ChainACSoftcap, 200)
RULE_INT ( Combat, PlateACSoftcap, 300)
RULE_REAL ( Combat, WarriorACSoftcapReturn, 0.45)
RULE_REAL ( Combat, KnightACSoftcapReturn, 0.33)
RULE_REAL ( Combat, LowPlateChainACSoftcapReturn, 0.23)
RULE_REAL ( Combat, LowChainLeatherACSoftcapReturn, 0.17)
RULE_REAL ( Combat, CasterACSoftcapReturn, 0.06)
Basically, more AC and HP is always better, don't think the soft cap means you should stop worrying about AC. On live during the eras with these caps I think most warriors went with 1 AC = 5~13 HP. And for reference on live now a days they usually use 1 AC = 10 HP