Here's 2 things that have just recently grinded my gears:
1) When it's called out in party chat that the monster you're fighting just used Invincible or Perfect Dodge and people proceed to waste their TP.
2) When you're at a campaign fort and there's very little people and some idiot always stands at the fort in the path of the incoming monsters and they aggro the whole group and run so that when they get killed they all aggro other people. Instead of doing the smart thing and letting all the monsters attack the fort so you can pull them off one by one.