How do you tell someone politely to stop pulling the mobs and to attack your marks?
I feel that even a chimp would notice the big numbers on the monsters head and associate them to some kind of order, but the ones that play this game, when they see a big 1 and 2 on a monster's head, well they attack the one without any number on their head.
They also run ahead and pull, making the task of establishing threat harder.
But my new favorite one is the healer that runs up to the monster I'm tanking to cast Fluid Aura on it, as if it wasn't already complicated to try and keep aggro when everyone is doing whatever it is they want to do.