Fixed gear, skinny tires. I'd probably be better off with cyclocross tires, but if you're optimizing for what you're going to be riding on 90% of the time, for

Fixed gear, skinny tires. I'd probably be better off with cyclocross tires, but if you're optimizing for what you're going to be riding on 90% of the time, for winter commuting 90% of the time it's dry/wet pavement or hardpack snow. If the fatbikes give people confidence to ride in winter, more power to them, but personally I think it's overkill. For the cold you've got to keep your extremities protected. I've got big dumb loose fitting mitts that slide on and off easily, and really warm socks. For my core, though, sweat is more an issue than freezing your ass off. If you dress to be cozy for the first 5 or 10 minutes of the ride, you'll be sweating once your blood gets flowing.