If nothing else, I think this sort of in game grading system would remove the human element of the equation. And I think we can all agree that it's the human element that causes most of the problems. Both people with the view that "you're doing it wrong" when maybe you really aren't for the level of content you're doing and people with the view that "I'm doing it right" when it could fairly easily be done better end up lost in mixed messages and missed communication with the way things currently are.

At least if it was the game itself that said "Hey, you got a B grade here. Maybe you can do better." it would be a completely objective way for the message to be put across. There's no room there for people to get hurt feelings over nothing or for someone to be an elitist jerk for no reason. Of course, this would also require some sort of quality training tools in game (maybe even mandatory every 5 levels or so as part of class quest progression) to back this up so that players aren't expected to go outside of the game and basically do homework.