EDIT:dammnit wrong forum, move to General

Wondering about the difference between road and mountain bike frames and why..

Gearing, tires, and brake type are obvious enough. But what about the actual 'geometry' differences?
Are these differences significant or is it really just a matter of the rider's posture?
and if it's only posture couldn't adjustments to seatpost and stems be used to cover such differences?
ie, can you make a road bike out of a mtn bike frame just by choosing a shorter stem, etc? or is there something really inherently different in the shape of the frames?