HP, especially later levels like that. My character concept is actually built around that after several players and myself remarked that you have to spend an extraordinate amount of character resources to get even passable AC vs. big monsters. Feats, class levels, magic, etc. Even then, the way Hit Dice and Strength and Size interact, even relatively low-CR monsters which are large are going to have their HD and Strength so pumped they blow through your armor.
Then there's Improved Grab and the Touch AC that just negates more than half of whatever you can try to do to get the AC.
My character, I tank AC as fast as I can dump it for every point of increased damage I can get ... Rage, Charge, Enlarge Person, etc, and dump as much into Power Attack as you can. The monster IS GOING TO HIT YOU ... so I've found it's much better to kill him before he can kill you.
I'm playing a Bbn/Cleric and the focus of my clerical spells is usually non-AC-type protections ... Energy Resistance, Death Ward, Sheltered Vitality, Immunity to Poison, DR, etc. Stoneskin and Righteous Might are really great because there's no way I can keep from getting hit, so it's worth it to mitigate those hits as much as possible.
Works great, really ... gotta watch out for the targeted dispel magic, though. Especially when they come three and four at the time in a round.
--fje