Biking is by far the most fun way to commute when the weather is clear, but in the winter you may be biking in darkness both ways and the weather is more likely to be rotten. Safety and visibility should your chief concerns when choosing to commute by bike at night, in the rain or in temperatures below freezing.Just the other day I took my first spill in years. It was a light rain, and I was going slow, but rounding a corner I crossed a crosswalk, and both wheels touched the painted line at the same time while I was leaning into the turn. Suddenly my tires, which had been gripping the road so faithfully, lost all traction and I found my face in the asphalt. I wasn't injured, but it was a shocking reminder that biking in weather is different: it requires more attention and caution.
I like to ride fast and hard, but when the road is wet you have to slow down, especially on turns: use the handlebars a little more and lean a little less. If the temperature drops below freezing, you can get ice on the road, which is the same problem but much, much worse.
Closer to the planetary poles, the days get awfully short in winter, and many people will find themselves going to work before sunrise and returning after sunset. If you bike, this essentially means biking at night both ways. Some states (Oregon included) have laws that require bike lights, but most urban cycling veterans will tell you to do much more than the legal minimum.
The more lights, reflectors and LEDs you have, the more likely a driver is to spot you before there is any threat of collision. Also, front and back lights are essential but having plenty of visibility from the sides will keep you from getting sideswiped in an intersection. Spoke lights can be as cheap as $5, and they create arcs of light when you are moving.
High visibility clothing and self adhesive reflectors are a must, no matter how dorky you look. I like to wear layers that I can easily take off and stuff in a bag, so the vest my In-Laws got me is perfect.
There will always be accidents. On our last trip to Seattle Courtney's family all admonished us with stories of a cyclist, covered in reflective gear, who was hit by a drunk driver in Kirkland recently. The one thing you can never control is the other people on the road, but the same is true in a car or on a motorcycle. It is best to cycle on routes that other cyclists use. People only look for bikes when they are used to it, so the more there are on the road the safer it usually is.
