I really liked Clancy’s comment – worthy of a post:
Not everyone whose number is retired has to be in the Hall of Fame (heck, the other team in NY practically retires your number if you successfully bunt a runner over to second).
It’s more about what the player meant to the team, isn’t it?  Keith Hernandez turned this franchise around, made it take itself seriously again, truly led the club in word and deed.  How many times would you see him walk to the mound in a tight spot and not just offer empty words of encouragement but emphatically challenge the pitcher to get that guy out, or find him in the dugout talking to one player or another – pitcher or everyday player – about the game, a situation, an at-bat? He challenged this team to be better, and they listened.  I consider him a leader in the way Gil Hodges was a leader, on the field and in the clubhouse (and to also argue time of service for number retirement, Hodges only managed for four years).
It’s not just about numbers one puts on the board, or about being a Hall of Famer, it’s about what you did for your team; I think Hernandez was the heart and drive of the Mets during his time here, and his number should be on the wall, without question.