This may be the first of several posts, depending on whether or not it spurs a discussion.
I've been pondering this question a lot lately, in part because we've had discussions at work about it. It seems like the U.S. is reaching a point where there might actually start to be demand for a commuter bike... but the question is, what makes a good commuter? It seems to be a tough answer to pin down, honestly.
The short answer is that ANY bike can be used for commuting, and that's proven every day. On my ride to work and in the shop, I regularly see a wide array of bikes being used to transport folks to their jobs. Pretty much everything from a coaster brake balloon-tire bomber to a high end carbon wonderbike gets used for commuting. And the owners seem at least content, if not down right happy with their choices. And there's nothing wrong with that... ride what you like, especially if it makes you more inclined to ride.
On the other hand, there are some features that I believe can enhance the utility of a bike for commuting purposes, and can make for a truly practical transportation vehicle. And in my opinion, it's all about utility and practicality and comfort in some combination.
To start with, I'm a big fan of larger cross-section tires. That is, larger than the typical go-fast road bike has (23 - 25 mm)... something along the lines of 28mm to 38mm seems ideal for pavement only commuting, and 38mm to 48mm for mixed surfaces. Why so fat you say? Don't those big ol' tires slow you down? Truth be told, they don't slow you down the way you think. Some tests even indicate larger volume tires can be as fast or faster than skinny tires. I won't wade into that argument, but I will tell you that in my experience, for commuting purposes, the wider tires are a net plus. The larger volume absorbs the shocks of cracks, rocks, potholes, etc, protecting both you and the bike from the impact. In particular, you're a lot less likely to damage the wheels with larger volume tires. This saves you money in the long run and makes it more likely that you'll get to work on time every day.
Second, a comfortable riding position. This means many things to many people. What is comfortable to the experienced roadie may be torture to the more casual rider. Generally speaking though, you want a position that is more upright than a typical road bike. This allows for greater comfort, and decreases the need for specialized cycling clothing, as well as generally making it easier to look around while riding, keeping alert to traffic conditions. A more upright posture generally requires a wider saddle than the stretched out racing position, but that does not necessarily mean the fattest saddle you can find.
Third, the ability to carry things ON the bike, instead of on your body. Now plenty of folks carry their commuting load in a back pack or a messenger bag, and some even prefer one or the other. Most riders, however, will find the ride more pleasant if the bike can carry the load. The most basic option would be a saddle bag or handlebar bag, for smaller loads, but those options are pretty limiting, and I'm of the opinion that the key to a good transportation bike is versatility.
Another good option is a basic rear rack, much like that used for bicycle touring. This can be used to support a wide variety of bags, from the basic "rack trunk" to "panniers", a style of bag that hangs on either side of the rack, lowering the center of gravity and providing a LOT of capacity.
Or perhaps even baskets, not widely favored in the US these days, but amazingly versatile. You can haul a lot of stuff in a basket, and they have the virtue of handling oddly shaped items better than most bags. There are even some that detach quickly and easily, allowing you to carry them in to a store for a quick shopping trip, for example.
Finally, and to many this is a shocking idea, fenders. You may have noticed that all of the bikes pictured here have fenders. Very few cyclists in the US even think of putting fenders on their bikes, and most modern road bikes don't make it easy or even possible to mount fenders. But take my word for it, they vastly improve the versatility of your bike, and make it possible to ride in virtually all conditions. You'd be amazed how much they help to keep you dry and comfortable in rain, as well as protecting your bike's vital parts from the ravages of weather. So forget the fact they don't look "cool" or fast... they are practical, sensible accessories that allow you to ride your bike more often, more comfortably.
Well, that's it for this installment. Feel free to comment, and perhaps this can become a conversation. If not, I'll just spout more of my opinions on the matter soon. :-)