Ok, it's all been said before, but I'll bite.
Today was about 20 degrees. I was very comfortable with a short sleeved wool T-shirt, a Land's End ski jacket, neck gaitor (up over my mouth because cold air gives me the coughs and my ears get cold), synwool lined leather work gloves, bike shorts, synthetic long underwear, jeans, wool socks, shoes. Rocking out in the snow for 3 hours. Moisture management (read, not getting drenched) is usually my biggest issue.
I'm excited, my commute tomorrow is supposed to be in the single digits. MORE WINTER!!
Ride on.