The OP more or less acknowledges it, but the "perfect" winter commuter bike depends on where you live, the weather in a particular year, the length of your commute. That is, there is no "one size fits all." Given that, for me the ideal winter commuter would have the following features:

- Fenders
- Tires at least 28 mm wide, with reflective sidewalls
- A great light system, front and rear. Could be rechargeable LEDs or dynamo, but LED lights with long run times work work best for me.
- Seat bag or rack top bag with plenty of room for clothes
- Drop bars, because it's windy here in winter
- Compact double or triple drivetrain
- Canti brakes with Kool Stop salmon pads

Frame material is unimportant, but for me steel provides the most bang for the buck. Clearance for fenders and larger tires is essential, but that can be achieved with a touring, sport touring or cyclocross frame.

To be honest, the ideal winter bike is close to the best all-year bike around here because winters are relatively mild in NC. The only changes I make in warmer months are narrower (and lighter) tires and fewer lights.