1) People who whinge after you stab them
2) PPSC (maybe gunboat PPSC is passable though)
3) People who don't finalise in gunboat/anonymous
4) Those who go CD, but then occasionally pop-in to destroy your plans while ignoring all your communication
5) People who hold grudges. In one game, a 1901 Ven-Tri move sent Austria on a crusade to take me down while effectively handing all his territory to Turkey. Not. Happy.
And... for something a bit out of centre field... People who have 1000s of points but then play 5

games and then they have the temerity to whinge about people going CD. If you've got the points... USE THEM!!!! (and save our ears from your criticisms of a system you don't use to its full potential)
Oh, I'm on a big rant now. I hate people who bag Kestas for not responding within 0.34 seconds every time they forgot to read the rules. This site is not run for profit - we should be encouraging mods/Kestas and being supportive of all the time they put in. ("Thanks guys" (and/or girls))