The trouble with folk as marketing category (the 'Mumford' approach) is that it's a label without a definition - and that means that you could find two songs, one labelled 'folk' and one labelled 'pop' or 'rock', and be unable to explain to anyone why one is called folk and the other not.

We can say, more or less, whether a song is traditional or not, and we can dig around a bit in the traditional repertoire and identify different flavours of folk - this song escaped from a parlour songbook, this one's a music hall number, this one was circulating on broadsheets when Pepys was writing his diary, and so on. (And, incidentally, if you can identify the song itself as traditional it ceases to matter what it actually sounds like - ambient trad, electro trad, death metal trad, go nuts.)

What makes a newer song sound folk-like is another question. Some people set words to traditional tunes, or actually emulate the style of traditional songs; some people hear 'folk' and think 'protest song'; and lots of people think James Taylor writes folk songs, so that's what they try to emulate.

Ceterum censeo the Bad Shepherds delenda est.