I will drop a group if players are playing their job poorly AND refuse to take any sort of constructive criticism, especially if Im being nice about it. Ninja's not using poisons or Huton, Monks not aware of positionals, BLM's spamming fire 3, etc etc. I always try to let a player know as nicely as possible that they can be making better use of their skills. I dont expect anyone to play perfectly but I do expect for you to have an understanding of your job and atleast make use of all the skills and tools given to you, or atleast be willing to improve when someone offers you advice on how you can play your job better. I can understand if someone is a dick and says "HEY U, GIT GUD OR GIT OUT", but people who can't take a generous suggestion are what make me drop groups.