Oh yes, stat-based gameplay. A long standing pet peeve of mine that has gotten countless goats from me. It's somewhat forgivable in games where you can't actually aim yourself (see: Fallout 1/2), but it can still be dreadfully annoying when you miss a 95% chance-to-hit shot twice in a row. It's beyond the players control, and that's what really gets me. If the computer decides that you are to fail, then you fail. No amount of skill is going to save you from that, which is why i typically avoid games where player skill falls secondary to arbitrary numbers. The most jarring example i can think of is Morrowind (sorry guys, i'm sure it's a great game but that accuracy thing killed it for me), as has been mentioned. If i swing my sword into someones face, then i damn well expect him to be hit by it!

Also, auto-attacking. I don't like pressing a button and then have my character start swinging ad infinitum, i'd much rather be able to move around dynamically and launch my attack when it's most opportune to do so. I don't mind cooldowns as long as they aren't ridiculous, because i still decide when to launch the attack again.

Basically, most things that take control away from the player, really. Stunlocks very much included. Also, that thing in many FPS's where your crosshair jumps up after getting hit. I know it's realistic to be flinching from the bullets hitting your face, but god DAMN it's annoying as well! Realism should not take precedence over fun. I mean sure, go nuts with the realism if it's fun, but don't skimp on the fun to make it more realistic.