More thoughts, back on the original subject:

To define a folk song, you first have to define the folk group. In my one semester of folklore in college, a folk group was very broadly defined - any identifiable group of people sharing some common characteristic was acceptable. For example city dwellers, computer geeks, and even a single family could be considered just as much a folk group as Appalachian mountain folk, Irish emmigrants, or eastern European Jews. Each group would obviously have their own folk tales, songs, crafts, foods, or other traditions. And the methods of dissemination can be different. For example I (being a computer geek) did as the semester project a collection of computer-related folk stories/materials. Mostly they were things that had been disseminated by copy machine, in those days (1980). Now fax machines, e-mail, and the internet are used. For example, the various virus warnings posted on the net and sent by email, almost all hoaxes, could be considered a form of folk tale. So it's pretty tough to exclude much if you allow any folk group in. For example, Wolfgang's hated football songs definitely qualify as folk songs for the folk group "German (or European) football fans". Maybe we need to work harder on which folk group(s) we are talking about.

My personal candidates, and what I have seen in DT for inclusion, would be British Isles/Celtic, Appalachian, Cowboy, African American (blues/work songs/spirituals). I would leave out singer/songwriter unless it was a song I liked. And there's the problem.