Slick tyres (or some semi-slicks running at maximum pressure) will make a lot of difference when riding on paved surfaces. New tyres, and possibly some full-coverage mudguards will turn your mountain bike into a bike for the road (not a full-on road bike, but pretty good.) I commute on an ex-mountain bike, and like the upright position for riding in traffic. For longer rides you may wish to eventually add some bar ends or switch to a different handlebar style for extra hand positions.