A couple of the posts above get to the heart of the matter -- it's not so much the bike as the setup. A cheap, steel bike boom "road bike" frame can handle most commutes. But depending on the conditions, the original tires, seat, handlebars, and fork may not do the job. A lot of mtbs are already setup fairly well for commuting, they're abundant/cheap, and a set of slick tires may be all it takes to get a decent commuter setup. mtb frames are perhaps beefier and heavier than most commuting applications require, but if you aren't lugging the bike up stairs, who cares? All that said, I sold my mtb because I prefer my steel 80's road bike, but I think for most commuters the road bike requires a lot more part swapping than the mtb.