I think it varies a lot depending on the specifics of your situation.

For me I have a 12 mile one way commute on a hilly high speed route. I don't think I would change much from my current set up. Steel frame cyclocross with fender/rack lugs (Surly Crosscheck) I ride it with fenders, but use a Carradice saddlebag instead of rack/panniers. Love the drops for the different hand positions and ability to get more streamlined in the prevailing afternoon headwind. 32 spoke wheels for strength and 700 c 25 slicks. Canti brakes, but wouldn't mind switching to V's. Couldn't live without the derailer cause of the hills ... I run out of gear for the long downhills but sometimes wish I had a triple for the steep uphills when I'm tired and loaded down going into the headwind on the way home. (36/48 with 12-25 cassette)

It's a nice setup and I sometimes pass dedicated road bikes, but if I couldn't store it in my office I probably would get someting a little more beat

PS I love the Carradice bag ... tons of room, even for a few assorted files. I carry my clothes and leave shoes at work.