I recently had a label or PR guy call me about a band he's working and he was rattling off the names of a bunch of mainstream country guys he'd opened for and I was trying to explain that we are roots and alt-country focused, not mainstream country focused but that mainstream artists such as Jamie Johnson and some of Dierks Bently's work fit in. He seemed to understand the distinction and thought his artist would be a good fit.
I watched a couple videos on You Tube and it forced me to spend some time thinking about what the difference is and how I could explain it to someone.
The music edged on that slick over produced sound and as soon as he started singing (in one of those seemingly forced deep baritone voices) it was about trucks, God, country and girls (and of course he was wearing the obligatory cowboy hat). I'm wracking my brain trying to remember his name. If I think of it I'll post the video on this thread and you all can decide if you agree about whether he would be a good fit here.
To me, as an "outsider" to the "scene" aspect of it, so to speak, I view mainstream country currently as including only "Country pop" (standard pop music using "country" trappings such as pedal steel, cowboy hats, acoustic guitars, etc) and sometimes "very well-established so-called Outlaw country & Bluegrass artists that have garnered large fanbases".
Anything country or roots influenced that is DIY or on an indie-label, or has a Purist/Traditional (non-pop) focus/song structure, or where the artist(s) are not quite "pretty" or "normal" enough to fit in with the current "rural super-model" mold of Country Pop fits in Alt. Country to me.
I think songs about "trucks, G-d, country and girls" still fit into alt-country too, as long as they are honest songs played by people who aren't millionaire super-models.