Anyone who's massively undergeared for the content (wearing level 25 gear in a level 40 dungeon...) or just blatantly ignoring mechanics doesn't get a commendation. Past that, I try to pay attention to who's doing their job, but that's a little rough to tell without keeping a close eye on them at the detriment of my own role. If all eligible folks appear equal, I go for tanks, then healers, then DPS.