"Obviously you must keep it dry"
What
isn't so obvious is how ridiculously easy it is to get down damp from sweat if layered under the most "breathable" of "waterproof/breathable" jackets.
I go with a midweight fleece on my cool weather section hikes. If you have sewing skills, you can buy one from a local second hand store, preferably with a hood, and cut it to your measurements so that there isn't any extra fabric to carry.
Fleece is relatively heavy for the warmth that it provides when dry, but when wet, it can be wrung out by hand (no light touch necessary as with some ultralight synthetic filled garments) and be put right back on.
Try that with down.
(all it does is ensure that whatever part is wet gets transferred to any part that is dry, so that the garment becomes uniformly
damp ).
For extreme cold, I bring down, but I pack it with my sleeping bag and it doesn't come out in damp weather until secure shelter is handy.