I'm the sort of commuter who likes to wear normal clothing on my ride, and these choices reflect that. I really feel like for any possible trips in this town you don't need fancy high-performance clothes: in my mind, cycling is just a way to get there, not the end goal.
Presented, then, for your edification, my list.
Things that go on you (I've bolded the cycling-specific items):
- Thin wool cap: keeps your ears and brain warm. SmartWool makes a nice merino wool hat that you can wear under a helmet or by itself when walking.
- Windbreaker: if you just take it easy, a normal sweater under a normal windbreaker is plenty warm for -5°C and below. Remember, you're exercising.
- Bright reflective vest: so you can wear a normal-looking windbreaker.
- Wool scarf: to keep your face warm when it's really cold. Also looks stylin' when you're off the bike.
- Lobster-claw mittens: These are mitten/gloves, with only two "fingers" and a thumb. A good compromise between warmth and mobility.
- Long johns: if it's really cold out, these keep your tuckus warmer. I haven't worn them in years but I may start to.
Things that go on the bike:
- Lights: to see and be seen. Check out the photo below of my battery-powered LED strand. Everybody notices my two front lights, one rear light, and string of colored LEDs.
- Reflectors: these work even better than lights when headlights hit them (which isn't all the time). I have 6 rear reflectors (on seatpost, rack, fender, two panniers, and under-seat bag) or 4 side reflectors (one on each wheel and each pannier).
- Cage pedals: so you can wear whatever shoes (or boots) you need for walking. You may as well put reflectors on these too: they're cheap and my tests show they're the most conspicuous reflector when viewed from the front of the bike. After years of fiddling around with pedals, I've decided that commuters just don't need toe clips, much less clipless pedals.
- Fenders: they keep freezing muddy water off your legs.
- Fat tires: studded tires are great, but also expensive. Fat tires are the next best option. Just take it easy.
- Panniers: they hold your stuff and provide another reflector location. I have one traditional zip-up pannier with coats and an insulated coffee mug, the other side is a folding grocery pannier that carries my lunch and whatever else I tote around.
- Rear rack: it holds the panniers to the bike.
- Bungee cords: to hold gift boxes, packages, etc. to the rack. I recently acquired a motorcycle "helmet net" which so far has been superior to bungee cords for the few things I've needed to tie down.