Primarily effects like silence (in most games). Poison, stun and confusion are annoying, but I can still forgive them for being indiscriminate. Silence, however, usually only affects magic users. Why the balls is that? Why doesn't it affect any sound-based ability, like a warrior's shouts in a certain mumorpuger? Or a bard's ditties? I demand equal rites for magic users!
Some other effects I really hate are any effects related to hit and dodge ratings, and healing/regeneration used by enemies (run of the mill mooks don't matter as much, but healy bosses are the worst), especially when combined. Fuck you, Muk. Fuck you and your Black Sludge [http://bulbapedia.bulbagarden.net/wiki/Black_Sludge#Black_Sludge]/Minimize [http://bulbapedia.bulbagarden.net/wiki/Minimize_%28move%29] combo.