Mountain biking is not riding said labeled bike on a smooth, dirt path. That's what a hybrid is for. I read people saying things like "Mountain biking is different for everyone". Sure, if you want to make yourself feel better, but there was a whole "trekking" group set designed for the "rigors"(

) of fire roading and hybriding.
What is the obvious difference between urban mountain biking in the true sense of the word, and just commuting? Technical riding.
Smooth singletrack is mountain biking. The narrow and winding trail adds a small amount of technical challenge to the inexperienced rider, but a wide, gravel path is just hybriding.
I think we should invent a new marketing label to the sport to increase marketing revenue.
Remember the Klunkers movie? Those guys back in the 70's saying: "Let's go Klunking." or "Let's go on a klunk."
"Hey guys! Go grab your helmet mirror! Let's go hybriding!"