My commute is short; 6 miles round trip. This affects my opinion. I also have access to showers and lockers at work. So for me:

• Fenders - I live in the PNW, so fenders are necessary.
• A head light - I often work late and in the winter darkness is ~4:30pm.
• Reliable - I only commute by bike, no car. This means my backup is a bus/max or walking.
• Light and fast - I ride with guys from work 2-3 times a week during lunch. They shouldn't beat me too easily.

I use a backpack for carrying gear/clothes. I have carried up to 40 lbs this way without problems. I have been commuting for four years by bike but am only now getting serious about cycling.

-Duncan