After last year's polar vortex, I have a new record low riding temp: -16 °F/-26.7°C. It is just -tiring- to ride in that (though surprisingly, it doesn't require a whole lot of new gear.) So I took the light rail to work quite a bit last winter (I bought a house last fall, and my very short list of requirements included "less than 0.5 miles from a light rail station".)
Most of my observations from previous years stand. Wool is wonderful and I feel very very sorry for people who are allergic.
Ski goggles are still awesome, but over-the-glasses is still a pain in the patootie. At warmer cold temps, the heat from your skin fogs the glasses as soon as you stop moving. At colder cold temps, the fog freezes on your glasses lenses. Honestly, winter/bad-weather biking may be the thing that -finally- convinces me to get contacts.
1. YOU STILL NEED MORE LIGHTS. YES, YOU.
One in front and one behind is really not enough, and I see many folks with only one (front -or- rear) or no lights at all.
Really, really - people can't see bikes in the dark. They just can't - pedestrians, other bikers, -and- drivers. Even if you have reflectors, those only work when lights are being pointed at you. Active lighting matters.
I use a -lot- of stuff from Night Ize - mostly from REI, including the Spokelit:
I have SpokeLits in green, blue, and "Disc-o", on two different bikes. I also use their Helmet Marker, and lots of their small carabiner lights.
My headlight is a PlanetBike Blaze - and wow, they are on super-sale right now at REI, so if you're looking for one... I have had trouble mounting that on the handlebars of my hybrid that I use in the winter, but a little Sugru applied to the mounting rig fixed that right up for me.
My taillights are nothing to write home about; I think one is Planet Bike and one is something else.
I also really like the LightWeights stick-on reflectors - have applied them to some clothing that has been repeatedly laundered, and they're still stuck on tight. Also have some on various bags, and my helmet. My jacket has reflectors built in.
I still like the Novara Stratos pants I got a couple years back, but also frequently use a pair of old men's wool pants I bought at a thrift store instead of them. The wool breathes better, and keeps my regular pants about equally clean.
I waterproof leather stuff (boots and mittens, primarily) with Sno-Seal. Good stuff, cheap, really works, have used on hiking boots for years. Don't use on suede unless you want it to turn shiny.
3. Mitten liners
Finally wore big holes through the custom wool liners my friend Amy made for me a few years back; luckily my friend Jo is a professional knitter/felter who is whipping up a new pair for me as I write. Still a huge fan of my leather chopper mittens, just not the nasty liners they come with.
The camping/hunting toe-warmers are not as helpful at ultra-low temperatures as I thought they were the first year I tried them. I bought some insulated boots last year, and those helped some on the really cold days.
I cannot imagine that $350-winter-biking boots -really- keep feet warm in a way regular boots can't, but maybe someone who has money to burn can enlighten me on that subject.
I also can't imagine wearing -any- shoe with a bike cleat in the winter, or even toe cages. I use my feet for balance/speed control way more in the winter, cannot -imagine- wanting them attached to the bike.
5. Riding surfaces
You're actually better off when it's colder, because solidly frozen surfaces are pretty easy to ride on (with studded tires. Incidentally, I got three full winters of use out of that first set; I need to replace them, but I'm waiting for a big sale next month.)
Solidly frozen stuff does lend itself to skidding, even with studs, when you go around corners. And if you slam on your brakes hard, you tend to fishtail. It happens. There is a lot more slipping and sliding when riding in the winter, no matter what the surface - you just have to stay relaxed, don't let a little slip or bump scare you too much, and you'll be surprised how much you -don't- fall! (Also, even if you fall on ice, which sounds kind of scary and hard, it is usually less rough than asphalt, and you're wearing more clothes, so less road rash than in warm-weather falls.)
Salt is NOT YOUR FRIEND. Yes, it's corrosive, but that's not really it. Salt creates soft patches. The softer a surface is, the harder it is to ride on, and the more you slide around. Worse, salt facilitates the creation of ruts, and if it freezes afterwards, ARGH.
Elevation changes that run parallel to your wheels (ruts, or the edges of where a plow went, or the edge of a patch of thick ice) are dangerous, especially when there are cars next to you. Ruts, etc, can make you move up to a whole foot sideways without your control; if people are driving too close to you, pull over to let them pass, you do NOT want to get stuck in a rutted area with cars close by.
Elevation changes that run -perpendicular- to your wheels are FUN. Well, no, not potholes and stuff. But ridges of snow leftover from plows, or berms at the edge of a sidewalk are actually great fun to ride over. If they're soft, they are also way easier to get through than a long skinny patch of soft snow parallel to the wheels.
6. Lube & cleaning
Last winter is the first time my -brakes- froze solid while riding. This was mostly from grit & stuff freezing in the cables of the pull. Blowing on them thawed them out enough for a momentary fix, but I started carrying super-thick winter bike lube in my bag for slightly longer fixes.
The winter beater I was riding last winter was just really bad on a lot of dimensions, but at one point, my freewheel gummed up completely. I was able to fix it by mostly taking apart the back wheel, and dripping a lot of lightweight oil through it for several days, but that was another contributor to the light-rail usage. It's possible the freewheel wouldn't have gotten so bad if I'd been more careful cleaning the bike. Weekly cleanings of the drivetrain (somewhere indoors with heat) is pretty essential in the winter.
In conclusion: winter riding is not for everyone. I am still not an evangelist. But if you think you might want to try it, you may find it as fun as I do! (If you're a local, let me know, and we can go riding some time!)