Pete has hit the nail squarely on the head here

Today I rode in on my full suspension MTB with knobbly tires so I can hit some trails after work... absolutely fine on the road, just a bit more effort than my normal commuter. Ideally you probably want a rear rack, fenders and lights. If you don't intend to commute in the rain and the dark then you can do away with the last two, if you don't need to carry much then you don't need a rack. I think the important thing is to get out there and have fun, that can be achieved on pretty much anything with wheels, pedals, a saddle and handlebars My newest (and most fun) commuter doesn't even have gears