? Accuracy that is based on chance and not probability (Battlefield 2 bots that can spray fire a SAW or RPK while running, without aiming down the sights; use shotguns from over 100 yards away but still manage to hit target in head).
? Player detection based on range, with no other variables (walls, flora, direction, and height need not apply; see that broken piece of $%@# game MW2 where one plant may conceal you, but the one next to it won't; yet you can slide by unnoticed by staying close to a wall 3 yards away from an enemy and his dog, in the open, and avoid being seen).
? Stealth sections where the enemy AI is set to GOOOORRREEE!! (again, see that broken piece of $%@# game MW2, where if you bugger up a stealth section, guards climb over random level barriers and chase you down.
? Stealth sections where enemy detection is binary (seen or not seen, let us again look at that broken piece of $%@# game MW2...and kick it while it's down. The instant an enemy notices you, every enemy in the entire level knows exactly where you are, and starts spraying bullets and grenades at your exact position, despite the fact that you're behind a dozen plants surrounded by trees and other foliage, and would be completely invisible to any -real- eye.
To summarize, the kind of AI I hate is AI that developers don't bother with. If a developer spends more time rendering shiny bump-mapped surfaces on everything and less time making gameplay playable, I'm gonna take issue with them; for other examples, see that broken piece of $%@# game MW2.
*Editing to say that I think Crysis is the only FPS game to do stealth well (and no, I don't call Thief an FPS, I call it Stealth). If you're playing sneaky in Crysis, you can sprint between cover, bushes, branches, objects, or even just lay in the long grass and it'll conceal you in a fairly realistic way. The choppers tend to ruin your day with thermal targeting or whatever they use, but it's still a blast going all sneaky in the jungle. And at least I know that if the enemy spots me it was because I was doing a terrible job at being stealthy.