The number 1 overall pick in the 2002 NBA Draft averaged 19 points and 9 boards in his nine seasons for the Rockets. Between 2005-2008 he was one of, if not the, most dominant big men in the NBA.
Ming became a free agent on July 1st and it had widely been speculated whether he would retire. The 30 year old has played in only 5 games the past two seasons due to ankle and foot injuries. If he would have signed to play this year, it would have been in a limited role, certainly no where near the 37 minutes he averaged in 2007-2008.
Yao Ming's contribution to the game certainly has been felt as basketball has taken off in China, thanks in large part to Ming's popularity.