> Anybody who has ever tried to sing what we have always accepted as a folk song knows that they differ greatly from all other kind of song - it is those differences that should be discussed

It's not black and white: there are many colours.

"what we have always accepted as a folk song" won't have exactly the same boundaries for all of us, so let's focus on what I am calling the classic corpus, the material collected by Sharp et al. Within that there are quite different kinds of song: for example the bucolic "Colin and Phoebe" sort, the ballads about battles between Scottish lords or lairds, those about sea battles, those like The Two Sisters and The Two Brothers that are set in no particular time and place, etc. To my mind the differences between those kinds are as great as between them and songs from the music hall.