Tanks marking past level 30 or so instead of just attacking whatever the dps are attacking.
Tanks who pull one trash mob at a time in CT of all places, because 24 people is just not enough to kill two imps.

White Mages who use Medica II, then immediately spam Medica til everyone's at full health anyway

Summoners who position their pets in front of bosses
Black mages who use sleep, ever, for anything.

People who stack for weights pre-heart on Titan, die, then tell everyone to stack even tighter so they have even less of a chance of surviving if they don't dodge in time.
On the same topic, people who complain that we're not stacking tight enough when NOBODY is getting hit.