Throughout his career with the Yankees he was consistant and reliable. He rarely missed games and was a competitor. It wasn't his fault the Yankees didn't win a World Championship while he was there.
Mussina was one of the good guys in baseball. He didn't get involved in the drug scandals like Clemens, Pettitte, and others. As a Stanford graduate he was one of the smarter guys in baseball. Maybe that is why he retired instead of sticking around a couple of more years.

He didn't care about his stats, otherwise he would be playing a few more years to reach 300 wins. The 300 win mark is becoming virtually impossible to reach these days as a result of the 5 man rotation and pitch counts causing starters to be pulled early. Mussina gave up a lot of money by not coming back. Today it seems like it is all about the money for most players.
Mussina will be missed.