Lets see...

Tanks - If they can establish hate easily, hold it well, use cooldowns as needed to make my role, whatever it may be, easier.
Healers - If they keep everyone alive well without extreme values of overhealing, toss out some extra DPS when everything is alright and theyre clear to.
DPS - When they kill things without halfassing rotations, don't complain if i have to wiggle as tank to dodge AOEs, and are overall respectful of the other roles.
All - Respect is a big thing for me, so when the party respects one another and doesn't just cuss each other out over 1 tiny mistake, then i am willing to commendate someone. Then i use my judging criteria from there on to decide which i felt deserves it most.