So often I hear, "don't tell me you rode your bike today...". Here is what it takes me to commute year round.

temperature	attire
-10 to 32F	boots, J&G raincoat and rainpants, snowboard mittens (bring thin knit gloves as liners if needed), balaclava, 1 short sleeved, one polar fleece top
33-45F	boots, J&G raincoat, balaclava, 1 short sleeved,snowboard mittens (if raining: J&G rainpants and boot covers)
45-55F	sneakers, thin winter headband,J&G raincoat, cotton t-shirt, thin knit gloves (if raining: J&G rainpants and boot covers)
55-60F	sneakers, work pants (if raining, non-work pants), non-work t-shirt
60F and up	sneakers, non-work shorts and non-work cotton top

Remember that I put fenders on my bike and they keep me from getting wet from below. Above 60 degrees, I generally don't worry about getting wet. A wind resistant jacket will keep me warm at colder temperatures and layering underneath means you can leave off a layer if the weather warms. The wonderful pit zips on my J&G raincoat are almost never zipped closed.