>>you must have some rationale that decides which are sheep and which are goats.
<<

I don't have any rationale. I tend to listen to a pop song and think "That would be nice done in a folkie style". Generally IMHO it would only be younger folk singers that would take modern pop songs and do them in a folkie style.

However on reflection, I suppose a true folk song would have lyrics that give a messge out about life in one way or another.

An example would be songs written about the demise of the fishing industry (very prevelant to the area of Grimsby (local to me)

But then, If I take John Conolly's Fiddlers Green thats about a fisherman who's time is almost up, and yet it is almost classified as traditional.

I don't know. You tell me.