What is Dad Rock?

Dad rock is ususally from the period referred to by historians as "back in the day," but it's not classic rock. None of

The Beatles
Jimi Hendrix
The Doors
Simon and Garfunkel or
Credence Clearwater Revival

are Dad Rock, even though there's a reasonable chance that any given Dad listens to several, if not all of them. If you want examples of Dad rock, look at Paul Simon on his own (I don't know if he was always Dad rock, but he's put out an album or two in the genre) or the 800 lb gorilla: Steely Dan.

But what makes those last two Dad rock? I'd really like someone to answer that for me.