My band has never been real big on dance tunes, but once people get up you have to do SOMETHING, so we've started paying attention to how danceable a song is.
Good dance songs that appeal to a wide variety of people (meaning all ages) in my experience tend to be of the lighter-weight pop variety with a swingy-boppy type beat. The teenagers and college crowd will think it's lame, but it's fun and will appeal to most of the crowd.
The song we play that started this...and which NEVER FAILS to get people dancing for some reason, is "My Music" by Loggins & Messina.
ObLaDi by the Beatles works well. Or any of the earlier Beatles stuff (e.g. Don't Bother Me, Twist and Shout, etc. etc.)
19th Nervous Breakdown by Jason & the Scorchers (not the Stones version)
Stuck in the Middle With You by Stealer's Wheel
Good Girls Don't by The Knack (GREAT song)
Bad Case of Loving You by Robert Palmer
Walk Like an Egyptian by the Bangles