FWIW, I'll add some commentary to Brambor's suggestions;
Besides the most obvious:
10 essentials type stuff
Totally agree, things can go south in a hurry in the winter.
I'd rather carry something else, like a pad and bivy sack. Unless you're in a gully or some specific area, you probably won't have the axe out anyway.
If you'll be above treeline. Most spots below can be by-passed or navigated with snowshoes. My next pair of snowshoes will probably be Katoohla MTN Step-In's, where the shoe and crampon can separate.
pretty well accepted as required. Be prepared to be able to fix "the usual suspects".
experiment with methods to prevent fogging. I'm still experimenting.
I've been having good luck with an old neoprene mask I used for skiing. Fit and moisture control are better than my nylon masks. Fleece hoods are too hot for me.
I like them, but don't believe they're required.
warm compartment for cell phone or radio
I keep my camera on a lanyard next to my chest. Other stuff can be stored with a toe warmer.
Toe/Hand Warmers: Bring 2 of each and swallow your pride. If you're cold, these things work great.
Food you'll eat and fluids you'll drink: say no more.
Down parka with hood and Hardshell jacket: I've stopped wearing my softshell pants in winter. I wear a softshell top and mid weight shirt and capilene and/or fleece pants under my hardshell pants.
Wool Liner Gloves and Over-Gloves or Mitts: Burn your thin fleece gloves. As soon as they get wet, they're useless. Get a bunch of 100% wool army surplus liner gloves and wear those alone or under a shell. Bring a couple extra for yourself and your buddy with the fleece gloves.
GPS: They aren't fail or fool proof, but they are important safety gear above treeline, IMHO, in winter.
Excellent Physical Condition: I don't think being in not great shape in the other seasons is a problem. In winter slow, labored, sweating progress is dangerous for you and your group.