I become frustrated when minmaxing players are automatically presumed to be dicks, bad RPers, terrible folks, and so on.
I become more frustrated when minmaxing, you are approved, and then a game actively engages in a house rule overhaul to manipulate their internal systems into either flat-out removing the benefits of your minmaxing, or making what you decided to minmax into an actual method of cheating.
I become even more frustrated when the staff refuses to let you start all over in light of their manipulative house ruling immediately after your trip thru CG.
There is nothing more obnoxious to me than sitting on a character that has been effectively neutered, actually liking the character you made, story-wise, and having little reason to play the character because it's sheet is now either effectively a mess, or completely useless, or worst case, completely inappropriate to the game.
But the worst is when players automatically treat you badly when it gets around that you minmax characters. I've seen players engage in everything from rumors - "Don't play with them, they're just out to PK you, ruddy minmaxers!" - to outright complaints to staff that you should be sanctioned, punished, or even removed from the game because you 'ruin it for others'.
We all log in to have fun, and to be entertained. Sometimes players don't think that minmaxers have feelings too.