The biggest is a lack of DPS from 4 DPS players. Boss fights go on forever, the kolto stations can't respawn fast enough. I had attributed that to "not enough abilities to have a DPS rotation".
Or that I'm tanking as a sith sorc while trying to heal them- which, again, is a sign of lack of DPS, if they're not out-threating heals.
A secondary problem is not grasping basic mechanics, like kill the weakest/adds first, don't stand in fire, etc.