Kinda like asking the owner of most Sport Utility Vehicles or a Cross Over vehicles what's the difference from a true off-road vehicle or station wagon, isn't it? Just raise the body up a few inches, put on some sportier (and bigger) wheels/tires and you're done, maybe throw in an all-wheel-drive system to make yours unique from the neighbor's vehicle (not necessarily a 4WD system).
I always thought they were a cross between a road bike and an urban bike of years-gone-by (Schwinn Collegiate & Suburban models come to mind). You combine the frame and seating style of a road bike with the larger tires and flat handlebars of an urban bike, and there you have it. Maybe throw in some design characteristics of the old rigid MTBs of the past, (as you suggested), and a touring bike for added 'street credibility'