I think there are two ways to grade, so I will hand out two sets.
The first way is the "Kindergarten Method." In this method, players are graded essentially for effort and relative to their own potential. This seems to be the way most players are judged by "Fan Masses." By this method, if your natural born ceiling is considered to be, say, a 60 and your on court contribution is a 60, you get an A+. By this method, if your natural ceiling is perceived to be 100 (doesn't matter that it IS, just matters about perception), but you only play at the level of an 85, you get a B. This despite the fact that the second player's on court contribution to actually winning is much much better than player A. This, generally, is the logic behind Rose over Howard or Lebron for MVP and the constant overrating of our own Glen Davis.
Pierce B+
Ray B
KG A
Rondo B-
Davis B+
Shaq C+
JO D
Green C-
West B-
Daniels B-
Krstic B+
Murphy D
Wafer C
Bradley D
The second method would be graduate/professional degree grading, where it doesn't matter what your natural ceiling/limitations are, it's the result that matters.
For example, if I were commissioning a family portrait, I'd much rather pay for one done by a guy who doesn't always do his best work than a guy recognized as, by far, the single greatest handless artist in the world...a portrait painted by the slacker is just going to be better hanging on my wall than one that is great...for a painter without hands.
So by this method, the player with a perceived ceiling of 60 who plays at a 60 gets a D- and the player who plays at an 85 of 100 gets a B. After all, player B just does more to get actual stuff that counts toward wins. Is player B more frustrating? Yes. Is player A more 'inspiring?' Yes. But player B just gets more done.
By the same token, I'm glad that Davis is phenomenal for an overweight, too short power forward...but the NBA doesn't curve scores for degree of difficulty, so the fact that there are dozens of low-paid players that do exactly what he does but make it look easier (or look like they aren't trying as hard) means Davis gets the same grade as them.
Pierce A-
Ray B
KG A-
Rondo A-
Davis C
Shaq B- (B+ when healthy)
JO D
Green C
West B
Daniels B
Krstic B-
Murphy D
Wafer C
Bradley D