Arbitrary mastery grades.
The final grade formula is inconsistent between games and roles. I've been playing Lux with consistent 3:1 kda and always in top 2 DPS, and have not gotten a S in the last 15 games. Plenty of time, 7/5 or even 5/5 adc/top, in the same game with little contribution received S. There have to be more going on than just kda, cs or dmg done.
I feel the way we're getting the grade now offers too little information. I know I've gotten an A, B or C but no feedback on how to improve. How am I going to do better without knowing what I did wrong?
Since Riot will not publish the rubric. I'd suggest a feedback system that points out the are of deficiency. i.e. "Your cs is lower than other players of the same position", "Your warding score is less than ideal", etc. That way we can start making improvement next game.