As a former Canuckistani infantry soldier, I took part in some "Winter warfare" exercises involving tents way up North in Sakatchewan ....
where there is only one wind break between you and the North Pole ...
and that wind break is a barbed wire fence ...
with one of the wires gone.
BRUTAL!!!!!
It ain't just the cold, it is the WIND CHILL.
As the author of The Budget Backpacker, I also tested out some of my own designs for down filled Winter camping gear and specialized cold weather techniques for my book.
After these experiences I moved to Vancouver Island, where it rarely snows, BUT the weather hovers around the freezing point with lots and lots of RAIN. In these WET Coast conditions, down is useless, a small tent gets almost as wet inside from condensation and humidity as it is out side, and a larger tent with adequate ventilation is a better choice. You trade off some heat retention with a larger tent, but the improved interior dryness is the advantage tgat you notice more than the temperature
Terrain and climate dictate gear, but I have found that for clothing multiple thin layers of wicking type fabric under a breathable shell are the best answer for both warmth and moisture control. I have general duty gear that is used for kayaking, bicycling, hiking, motircycle touring, etc. Not specialised or optimised for any one activity, but usable for almost everything.
PS: a while back I had a job at a Remote mineing camp wayyyyy up North in the interior of BC in December. Surprisingly, it was not that cold, with temperatures again just around freezing. Working outside, 10 to 12 hour shifts, I found once again, moisture control was at least as important as warmth. I carried a couple of extra pair of thick wool socks every shift, and often ended up giving them to crew members who had wet boots.
Thick wool socks ... a dozen pair are highly recommended