This is a very old (1989) article I read about "evergreens" - shows that just don't wear out in syndication. It's from the era when cable was still a gathering force, so it mostly focuses on syndication to independent local channels:

http://www.tomsoter.com/node/1425

I think its takeaway line, "a successful oldie must have strong storylines and well-written characters," still holds.

Sitcoms cited as evergreens include Andy Griffith, The Honeymooners, I Love Lucy, Gilligan's Island, Brady Bunch, and I Dream of Jeannie. Mary Tyler Moore, Mayberry RFD, The Lucy Show, Here's Lucy, and All in the Family were mentioned as not being evergreens.