I winter bike. The skinny-tire bikes are TERRIBLE. Forget them.
Mountain bikes are so much better. It's like the difference between slicks and snow tires for cars. I haven't tried the new fatties but then if your streets are generally plowed well enough- so YOU aren't plowing through- I find regular mtn tires sufficient.
Get a cheap, preferably free beater (garage sales, alleys, craigslist, etc.). Here in the Rust Belt the road salt will make derailleurs and such look like they've been on the bottom of a lake for 100 years in one season. Cover it all with lube and don't sweat it if you have to repair or replace it more often than you are used to.
Easily openable layers. And big pockets. You start out cold but it's amazing how hot and sweaty you can get when it's below zero. First I'm unzipping my jacket's shell partially, then completely, then unzipping the liner, then stuffing the hat and gloves in the pockets, until I finally arrive at my destination completely nude.
