This is a winter gear thread now?
I used to do a fair bit of trekking in snow progressing from the Mournes in Northern Ireland to the Alps and topping off with the Himalayas
Mountain marathons... Climbing Instructor...
This was all some time ago - been a bit of a city boy recently.
Much of this below has already been mentioned
and its not comprehensive - just some thoughts really.
Agree about the wide mouthed flask - or if you are like me, EXTRA wide mouthed :p Careful where you put it : "Funny tasting apple juice!" but that's another story...
Socks, socks, socks. Sleep with wet ones close to your body - they'll still be wet in the morning but at least they'll be warm. I never got on well with layering socks - so I just buy the heaviest most expensive hiking socks I can find.
Boots are critical - but I always seem to be cursing whatever I have on. Leather or Goretex - there's pros and cons to each. Never lost a toe to frostbite - but even the white ones hurt a lot. Sleep with boots in your bag rather than risk them freezing over night.
Gaitors. When I've not had them - I've usually regretted it.
Snow shoes could be a life saver. Wished I'd had them a few times. On one trek we could cover maybe 2k before the sun hit the snow (brutally cold though) but as soon as the sun came up we started sinking to our waist with every step, falling through into melting streams and bogs... hell. This was compounded by being at 5000m with giardia and wolves circling on the horizon. Actually the wolves helped me go on - if I had given up I'm sure they would have been on me pretty quick.
Same for crampons, rope, ice axe, dead man, ice screws etc - but that's getting specialised. Mission specific I guess.
Sandals - solid lightweight ones for giving your feet a break in camp/ at night, and also for e.g. fording a river where you don't want to get your boots wet but need to protect your feet.
A foot care kit (notice a theme here?) cold wet feet blister more easily. Long nails eat through socks. A great uncle of mine DIED of trench foot in WW1.
Waterproof stuff sacks - for your socks amongst other things
More socks. I don't really carry spare clothes - I'd rather be smelly than hump the additional weight. But socks - such a moral booster - nearly as good as a cup of tea
Suncream and goggles. Especially at high altitude where the UV is stronger. Snow reflects like water - so you get double trouble. Had snow blindness on one trek - walked for three days with my underpants over my head - peeping out through the fabric. Hehehe... ouch.
Layers of clothing. Options, options, options e.g. it could be very hot in terms of radiant heat but with a freezing cold wind - requiring a thin but wind proof outfit. Then the sun comes down and its so cold you can't think. So, be in your tent, in your sleeping bag with a cup of something hot in your hands BEFORE the sun goes down... A blizzard is a completely different beast... full waterproofs, and layers... Tricky to get it right - especially if active then not, like on a multi-pitch climb, or playing hide and seek :p Has anyone solved the sweaty back problem?
Gloves. I seem to get cold extremities earlier than most. Tried lots and finally settled on a thin pair of thermal gloves inside a massive pair of fleece-lined goretex gauntlets. Its worth thinking through what might happen if your fingers are so cold you cannot feel them. On this basis I put extra large tags on zips for example. Also try e.g. opening a folding knife under those conditions. Check all your gear - can you operate it in mits?
Keep the weight down *doh*. Noodles rather than rice perhaps - they cook in two mins as oppsed to 10. At altitude boiling water isn't as hot... so takes longer to cook = more fuel = more weight.
Quality fuel container. One fall and its A) leaked all over your pack contents B) well... no hot food or drinks. Happened to me once - snapped the top off a plastic screw lid (cold makes some plastic brittle) Fortunately I had condoms... although my trekking partner gave me funny looks.
Pump-up muti-fuel stove - you need that furnace for melting snow, and they will burn kerosene, petrol, whiskey, perfume...
BIG stainless pot - again RE melting snow - otherwise you will be contantly feeding it just to get a mouthfull. Also for making a snow cave...
Water purifying tabs - at altitude boiling water isn't at 100deg so it may not be sterilising.
A reflective tarp - I used to have one that had the same footprint as the tent - got a lot of use for various things.
Sleeping bag. I used to use a down sleeping bag on the warmth to weight ratio basis. But it WILL get wet - at least mine always did - then its useless. Plus you need a better kip mat as the down compresses under your body. I've had one synthetic bag (Ajungilak 5 season) for twenty years now. Its still going strong - still warm if wet, doesn't compress so much under my body - but its twice the bulk of an equivalent down one.
Kipmat. Never got around to buying a self inflating kipmat - but always wanted one. But you need something - closed cell foam, is fine..
Tent - too much to say here and there's pros and cons of whatever design. I had a 2-man Wild Country Quasar that was pretty pricey but lasted me nearly thirty years. It went all around the world, jungle, mountain and desert, and weathered many a storm. Full sized doors at both ends plus mosi nets that were fine enough to even keep out Scottish midges. The two openings are amazing for controlling ventilation. A bitch to pitch if I'm honest. Finally threw it away with a tear in my eye just last summer. They still make them today with an almost unchanged design. Now with family we've moved on to the trusty old Vango Force 10 Mk5. Whatever - understand the different factors influencing microclimate. Sun, wind, reflectance, evaporative cooling, nighttime radiative cooling, thermal chimneys etc etc and how the landscape alters the ways these factors interplay Choosing where to pitch your tent could make all the difference.