I haven't played nearly as much as some veterans, but I'm quite experienced in the nerdier side of NBA statistics. So here are my suggestions after playing a few seasons
1) Adjust old player's FG% upwards.
99% of players from the 50s and 60s are useless. What's the point?
2) Decrease the cost of low FG% players.
Again, low FG% players are generally useless unless they don't shoot much and have great other attributes
3) Fix Free Throws
Players like Karl Malone and Adrian Dantley have their games based around getting to the line. You're killing a lot of players by cutting their FTAs in half (yet nash, peja, etc all get the to line normally)
4) Increase defensive effect on FG%
Everyone is running up-tempo, and FG% are higher for every player across the board. How many 140-135 games need to be played?
5) Adjust 3P FG% downward
The three-pointer is being hit even higher proportionally for some reason.
6) Allow coaches to determine usage
Assigning a guy to shoot higher than his usage lowers his efficiency and vice versa.
a) Teams with similar players will play differently based on coaches decisions
b) Certain players now become viable, especially with my second suggestion. You could draft Antoine Walker for cheap and play him as a point forward that rarely shoots, or Iverson as a defensive pick pocket that drives the lane and never shoots jumpers.
7) Rebounding is currently under-priced, adjust cost upwards