The problem with trying to come up with a general definition for "folk style" is that, at least as far as the singers of traditional ballads is concerned, it is all about individuality.

Listen to Granny Riddle, Maggie Hammons Parker, Sheila Kay Adams, Jean Ritchie, Dwight Diller, Addie Graham, Dellie Chandler Norton, etc.

If you can come up with a definition that manages to encompass all that you hear I'll be truly impressed.

I've noticed that when people wax enthusiastic about such singing they don't talk about the technical stuff--grace notes, melisma, pacing, rhythm, etc. They talk about the things Jerry mentioned--feeling, energy, etc. Things that I wouldn't ordinarily think of as stylistic.