"So, the serious question is, what makes a new song a folk song? Should it have a particular kind of tune? Must it carry an important message? Does it need to be about ordinary people's lives?"
as far as the uk folk revival goes, most of the songs that are considered to be in tradtional style, belong to a limited group of modes, so yes they do appear to have particular kinds of tunes, limited to certain modes, they do not all appear to carry an important message, and some are not about ordinary peoples lives.
for example ICARUS was not an ordinary person.