I very rarely give any coms whatsoever. But I do have some general criteria

1. You must do your mechanics correctly. Failure to do mechanics no matter what role unless you are new means you are disqualified for me
2. Your glamour must not be stupidly naked or mismatched
3. You must perform your job to a basic degree. For dps this means no freestyle samurai, cryo mage, "im a lancer main" etc healers is you must do dps during the 50%+ of the fight you won't need to heal and tanks you must use your mitigation and move the boss correctly
4. You must not be a lalafell, bard or blm. I have a hatred of all 3, the former are creepy, the latter caused me too many frustrations and jealousy over the years thanks to continuous buffs from yoshida and too many blm being lazy and not moving. Bards for annoying me in limsa or gridania

To earn the com as opposed to not being disqualified, you generally need to do something good that is head and shoulders above the others. Save the run perhaps, sac yourself to save the healer. Or even carry the rest of the raid because everyone other than me (and on the rare occasion when im not doing well, including me.