If I'm healing:

*If the Bard uses Foe Requiem while I'm spamming Holy or spells=Com

*Not that I need it sometimes but if the Bard uses Mage's Ballad=Com

*Bards who sit on full mp and never bother to sing anything even if I'm dpsing with spells or low on mp=no com

*If the Tank holds hate well=Com

*If the Tank actually uses Cover successfully on me=insta-com

If I'm anything other than healer:

If the healer just sits there not doing anything for the majority of the fight and not DPSing when there's cleary a good window to DPS=no com

It depends really. But that's my conditions for commendations.