I think you're trying to put to fine a point on it. Personally, I think there's too much division within mountain biking already. The equipment manufactures want to break it up into all these little niches so they can sell more stuff. But, honestly, each artificial division contains elements of the others. Downhillers still have gears because they occassionally have to pedal up a hill...poor bastards XCer's have shocks because they occasionally have to go down something steep and rocky. Everything else in between is just different shade of gray. Except maybe the numbskulls who throw themselves off cliffs...those guys are in a whole different world

As for trails being flat and unchallenging, any trail can be made challenging. All you need is speed. A flat twisty trail through the woods can be challenging as hell when done at 20 mph Especially if it has lots of nice limbo sticks.