Lots of good advice here because of the experience. Smatterings of preferences too! Like Nike, just do it.
I agree with most of the point Brainic what ever stated a couple of posts up.

1. The right size that will allow you to get to the right fit. This means you will have to find the right location for the saddle and handle bars.
2. Gearing - to get you back home
3. visibility - lights so you won't get hit as easily.

You didn't state if you had any biking experience in your past. If you do, that is great! I was a pretty intense rider in my early 20's then went nearly 30 years without really riding. At the time, we lived on a hill with a section of the road that had a 20% climb. I died trying to get up it on my 30 year old race bike, 39/21 was my low gear! For the first year, I used my MTB with road slicks. That got me into shape to the point where I could ride my race bike up the hill, newspaper boy approach.

The point is that everything changes as you do it. You get stronger, the bike becomes familiar. You find that what you thought you needed, you don't and what you didn't think you needed, you do! Gotta start somewhere, the basics is the best place.

Do you plan on doing any other riding than commuting?

OK we need some pics on this thread!

This is what I started commuting on in 2008. The picture is what I sometimes use today. I have over 3000 miles on this bike and just replaced the chain and rear block (cassette). It is my "truck" and my choice for bad weather.
[IMG]1997 Specialized RockHopper, on Flickr[/IMG]

I have since moved to a flatter location across the country and now have nearly 2000 miles on this bike I commute with.
[IMG]1991 Pinarello Montello 60 cm, on Flickr[/IMG]

With the RockHopper I can haul a lot of stuff, even with those smallish "Panniers". I take clothes to work on one day to cover a week or so. Then I can take the Pinarello the rest of the week. I use a backpack on the Pin and don't like it being there but I like riding the Pin more than the RockHopper! There is about 3 MPH difference in my 11 mile commute.

Oh and BTW. I believe I paid nearly $400 for the RockHopper in 1997, new last year model. I bought the Pinarello last year, for under $500 on CL. This picture is after tearing it down lubing and cleaning. New cables and bar tape is all that is different from purchase.
And the difference in weight? With all accessories removed from both, there is a 6 lb difference, RH at 27 and the PM at 21.