I used to be an avid 2K player and it is still my favorite basketball game (by far), unfortunately I just haven't had the time to keep up with the newer ones.
On the topic of ratings, I agree that to make it more realistic, they should base 'average' on 50 instead of, say, 75. This would make it impossible for anybody to get a 99, or even much less be able to score in the 90s. This isn't to take away from the great players, but to actually create a greater distinction between them and the ratings of regular players.
As for the actual grade of a 75 - this was D in my high school (under 70 was an F). How happy was I when I got to college and had a whole new scale of point with which to work (although anything under a C is useless in college anyway).