Road bikes are much more efficient. That's it. The frame and forks are lighter, pretty much every component is lighter, they have thin (usually treadless) tyres which are run at a much higher pressure, they're more aerodynamic (frame shape and the riding position) and have a gear ratios more suited to speed rather than mountain biking over tricky terrain. All of these factors add up to the rider being able to use his/her energy much more efficiently, meaning they can go faster/further. If you don't want a road bike, to get an advantage, I'd suggest getting some slick 26" tyres and some clipless pedals. edit - LayerWAN - Road bikes can have up to 30 gears...Hell Campag are bringing out 11 speed soon, meaning you could have up to 33 gears!