Having renewed my credentials with Mudcat, I found this thread, and have read some of the debate with interest and sometimes amusement.
Of course it is impossible to draw a line between folk songs and other songs (leastways everyone will have a differing opinion as to where the line should be drawn)

However, my wife and I have been playing a game for some years at festivals, folk clubs and other music events where the F word is implied but not used: - was that last song folk or not folk? - we don't debate - we just see if we agree - and mostly we do.

This activity has led us to try to crystallise our previously unstated criteria, and it pretty much coincides with the answer to the question; - did that song tell a story, and if so was that story of interest to anyone other than the author/performer? two yesses and it qualifies as folk

I expected that I would find something similar in this thread, but was surprised that there were no references to 'telling a story' in the bits that I scanned though!?

PS I looked up the plural of yes, and found a minefield almost as large as the above thread!