Great advice on here and similar to what I carry. I actually have a summer and winter kit, but the winter kit supplements the summer kit and is not meant to be used separately.
Off the top of my head the winter kit has a sleeping bag (to supplement the wool blankets and blanket pins in the summer kit), more fire building materials (lots more pjcb, sawdust and wax firestart brick, extra flare, more fatwood, another lighter, matches and a fat ferro rod), Snow Claw (to supplement the shovel and to aid in snow shelter building - hands down the best tool I've used for snow shelters), toilet paper heater (small coffee can with a roll of toilet paper in it and 2 bottles of alcohol), extra lithium batteries, more disposable chemical handwarmers and Heat Solution reusable handwarmers, axe (to supplement the hatchet and pocket chain saw), can of florescent spray upside down spray paint (for marking SOS in large letters on snow, although I haven't tested this out and I have a sneaking suspicion that I'll run out of paint with only one can), HEET, lock deicer, and a few other sundries I can't think of off hand. I'm also thinking of picking up a wood grenade-type splitting wedge to throw in there for a longer term survival situation.
This is a lot of crap but I'm in the mountains a lot and also drive from California to Utah, Idaho, Wyoming and/or Montana during the winter to visit people. Of course, I'm constantly tweaking it. Last Christmas i actually left a lot of it behind due to lack of space while driving to Utah and Idaho, and just KNEW we would need it, but we didn't (which was a relief and also a disappointment).