if solo backpacking in winter:
bivy sack, -20f sleeping back and sleep in your boxers and a t-shirt (plus all you other stuff)
if camping with others.. a simple three person tent. (remember that gear is considered a person). light is always good. next to floor space.
low ceilings can lead to frost buildup on the inside of the rainfly, it happened to me once on a particularly cold boy scout camp out.
then we get into the goose down vs Synthetic debate for fill of the bag.
down Pros: light, super compact, super warm with a higher fill power (800+) super breathable
down Cons: once it gets wet... it takes FOOOOOOOOOREVER to dry out. (at least the one i had years ago) the down will matte afterward and then get "cold spots" in the bag, so always store a down bag out of the stuff sack, and these get PRICEY.
Synthetic pros: dries out faster when wet, moderately light (around 3lbs), wont get cold spots, durable longer, just as warm as down, CHEAPER!
Synthetic cons: doesn't pack down real nice, kinda bulky, kinda heavy
i have been using a synthetic bag for the last 18 years or so. i store it outside of the sack hanging on a hanger. i am happy with the 0 degree rating is has.
as i think of other things i will add them to this