I find it very funny that you want a rating system while also saying that WHM spamming Holy is bad. Holy is easily one of the best moves on the class. For that one comment alone I rate you at F.
In all seriousness though, grading systems suck. I'm glad they are not in the game. They breed a toxic mentality and the fact that the game does not care is a good thing. Grading systems can't account for the individual perception of fun. If I want to commend a dps for dying in a hilarious way or telling a really stupid joke, I'm allowed to.