Before making any big investment on gear for commuting, try it once or twice with the gear you currently have. Or, ride to work and back on an off day to see it you still like the idea and think it's manageable.

Looking at bike racks at work, people are commuting on all different style of bikes. From re-cumbents, old steel MTB's, touring bikes, older road bikes, newer Ti road bikes, single speeds, and skinny tire racing bikes. Any bike that gets your from point A to B will work. As TSL said, find a bike that gets you excited about riding and make that your commuter.

My commute is 28 miles round trip. I'm more of a fair weather commuter. I work swing so always riding home around midnight, so gotta have multiple lights on the front and back. I've settled on a steel single speed with racks and fenders on a mostly flat route. When headwinds are strong, I would like some gearing, possibly my next N+1. I do find commuting invigorating with a great sense of accomplishment knowing I can get there and back without the truck. Plus I just feel better during the weak.

If you have nice safe route devoid of heavy traffic, that's also a big plus.