If you look at the list of top 25 greatest NBA players from FOX sports, not a single one is under 6 feet tall. In the top 10, the shortest guy is 6'6", while 7 of them are 7' tall.
I couldn't find a list for this season, but a list of 2013-2014 season had the NBA with 14 guys under 6' tall, of which, half were over 5'11". That is 14 out of 450 players. It would seem like one of the most important factors to being in the NBA is being about 6' tall.