I feel choosing a bike is a very personal decision based as much on personal taste as on functionality. There are several characteristics to consider, and the perfect commuter bike can't have all of them.
The first thing to consider is whether the bike is appropriate for you. Does it fit you? Does it fit your ride? If you are having to take off-road trails or have to jump several urban curbs, maybe a full-suspension mountain bike is best. If you have a long commute, a light, fast bike is preferred. If you're in a high-traffic area, you definitely want a bike that sits you upright. Since there's no such thing as a light, rugged, full suspension, fast road bike with cruiser handlebars, you can't have it all. You have to decide what your priorities are and get an appropriate bike.
If you already have a bike that you like, chances are you can use it as is, or maybe add a rack, panniers, baskets, whatever if you want to.
Another important thing to think about is security. If you can bring your bike into office or have some other secure parking opportunity, you can get away with a relatively pricey bike. If you're going to be locking your bike to a No Parking Sign out by the road, you probably want a horrible-looking beater.
Another consideration is how you will carry things. Some people get a backpack or messenger bag to ride with. This is a good idea if you're locking your bike up in a public place because then you take all your stuff with you. For me, the downside of that is I don't like riding with stuff haning on me. (You may feel differently; it's just a preference.)
If you want to carry stuff on your bike, though, make sure you get a bike that has mounting points for a rack.
Since I'm into value, my advice would be to do one of the following. Either buy a used bike or get a fully outfitted bike at a bike shop. Under no circumstances would I recommend going to a "big box" store and buying a bike that's been assembled by someone who knows nothing about bikes. Even for bike brands that sell at both the big box retail outlets and local bike shops, there is a difference. The stuff in the big stores is cheaper and lower quality because they are selling to people almost on an impulse basis. A bike shop, on the other hand, will help you figure out what your needs are and show you different offerings to fit that need.
Buying a modest-priced bike for commuting pretty much pays for itself. It's a lot cheaper than a car, and if your alternative is to ride the bus, you can save enough money in unspent bus fare to cover the cost of your bike.
In future posts I will discuss different types of bikes in more detail to help a new commuter decide what kind of bike is best.