If a party member did something that left a good impression on you, give them a commendation. If not, don't. The system doesn't need to be more complicated than that and certainly doesn't need a 'grading system'.

Plus, as a whm, the fact that you're speaking against holy spamming, one of our most valuable tool against trash in both damage output and damage mitigation via stun locking, makes me glad you don't get to rate your whm on performance <.<