As a tank:
1) Healer leaving during boss fight...three times.
2) DPS popping cd's on trash...on the mob you're putting aoe dmg only into.
As a DPS:
1) The plate tank needing on rogue gear
2) The healer not healing the dps on aoe heavy fights.
3) DPS leaving midway through a fight...on Corla.
And you wonder why Blizzard has to give silly incentives for people to do randoms