Several good answers here beerbellys. Now would you like the best answer? Look at everything. This guy would not meet half the criteria listed on this thread:
http://www.whatifsports.com/HBD/Pages/Popups/PlayerProfile.aspx?pid=1813158 due to his control, 1st pitch, etc. However, the fact that none of those numbers is actually bad has made him a nice #3 pitcher. Besides the numbers that affect his pitching, the 83 Health and 92 Makeup assures that he stays healthy and is showing zero decline even into his late 30's. Best of all, that 99 patience means he keeps coming back every year for a new one year contract, and the low 71 overall rating means he signs dirt cheap.
Disclaimer: I have to admit his numbers are probably helped out by playing in front of a stellar defense, which was the focus of this team.