Toadfrog, your definition is probably as good as any. I don't subscribe to it, however, because of its emphasis on simplicity as an essential element (or THE essential element) of folk songs. I think a lot of great folk songs are actually quite complex -- musically, lyrically, conceptually. So your definition doesn't work for me.

The definition that does work for me (which others are free to embrace or reject), is that the essence of folk music is its ability to exist and evolve without benefit of a larger governmental, corporate or pedagogical infrastructure. By this definition "classical" music (also a misnomer, but we'll leave that for another thread) ceased to be folk music when the pedagogues began dictating the rules, and the composers/performers became increasingly reliant on either private or governmental subsidies, both for the continued production of the music and to ensure that it could continue to be performed. This became increasingly difficult as the music got bigger and more complex (think of the infrastructure that is needed to support Wagnerian opera, for instance). Similarly, I would maintain that rock and roll began as folk music (taking Elvis' Sun Studio recordings as a starting point), but was quickly co-opted by corporate entities that sought to control the creative aspects as a way to ensure that their profits would continue to grow. Jazz began as folk music, but as the rules increased, it gradually drifted away from its folk roots, losing much of its audience in the process. All of these musics have occasionally received "folk infusions" in the years since they departed from their folk roots (the incorporation of folk tunes by Dvorak and Stravinsky, the punk-rock movement of the 1970's, various "world music" additions to jazz), but when these take hold they are often quickly incorporated into the music being made within the larger infrastructure. But I do believe that any of these musics can simultaneously exists in folk and non-folk varieties.

Submitted for your consideration. I know a lot of people have grown tired of these "what is folk?" discussions, but I find them to be among the more meaningful topics discussed on this forum.