I like that idea of "the folk idiom". Assuages the guilt of those who are not pure traditionalists (there are many of those purists out there--you better not be playing in a modern style around some of the more rabid of them. They'll turn up their noses in contempt.)
I do think it avoids definition. Can't say it's all acoustic, it isn't, though electric instruments are usually not the main ones. Can't put a year on it (like anything before 1940, for example--although I knew a couple of folksingers who refused to sing anything less than 50 years old.) A lot of campfire songs were written by the likes of Tom Paxton, Bob Dylan and Pete Seeger and such--folks who are still alive and writing.
(Check out Eric Bogle's "Traditional Folksinger's Lament" for a fun take on Dylan's influence on folk music.)
But there is a "sound" to it. As Schooner Faire says, "Fifty years from now you probably won't see guys in the bar standing around the old upright synthesizer singing Twisted Sister tunes..."
I play the stuff, but I couldn't define it. Not to mention that the songs change through something called "the folk process".
But the genre, if you expand it beyond just the traditional, is great to listen to, fun, thought provoking, and in too short a supply.
Let's get a grass roots movement going to start "The Great Folk Scare of the 21st Century".