re: What are some of the best players that never took advantage of their potential?

Len Bias was the first person I thought of.

After him I thought of several people who had good to great pro careers but they were cut short due to injury: Bo Jackson, Penny Hardaway, and Grant Hill.

Then there are the boneheads who couldn't keep their heads on straight and didn't try hard enough or got in trouble. I think there are hundreds of these guys.