Besides the points that Grim so clearly illustrated, I'd say that the one thing that can adapt a mountain bike really well to road riding is skinnier tires (and, if the frame will take them, 700c wheels). At one point, I had both a fully rigid Cannondale Bad Boy Disc and a full-suspension Cannondale Rush. I found that the Bad Boy's 700cx28 wheels and tires fit onto the Rush, so I took the Rush out a few times with them on. With its shocks locked out and with the skinnier, lighter wheels, the Rush rode almost identically to the Bad Boy.

Flat bar bikes with a more upright riding position aren't much of a drag for my short commute, but if I lived across the river, I'd definitely use a road or touring bike instead.