I just recently purchased a bicycle for commuting. I decided on a Surly Long Haul Trucker after riding several different bicycles in the shop. Personally I really wanted a bicycle with drop handle bars. That eliminated all the hybrid/comfort/city style bicycles and left Road bikes, Touring Bikes, and Cyclocross bikes.

If I had decided on a road bike I would have to get used to wearing a loaded backpack with a 15" laptop, and clothes for the day while riding.
On the cyclocross bike I might have been able to get a rack with panniers that would hold my things, but not have me striking the bags while pedaling. I cam very close to going this route!

Ultimately I decided on the touring bicycle because it offered the overall best platform for how I and where commute to work. Big panniers, hooked on a rack. Riding from the outskirts of the suburbs into a very small town in the rural midwest. I ride on county highways along side corn fields

The Cyclocross bicycle, and Touring bicycle easily accepted between 25mm and 40mm tires. I have 35mm Continental Contacts installed. They are pretty tank like, which inspires a lot of confidence while riding on rural highways to work. I can't count how many rocks that have fired out from under my wheels as if fired by a gun that I'm certain would have instantly flatted a true road tire. Finally the cyclocross and touring bicycle both had plenty of clearance for fenders. While I don't think they're absolutely necessary, fenders sure are nice when it rained the night before my commute and the roads are still wet.