What matters most when you need a warm night's sleep in the field?
Regarding a warm night's sleep in cold weather, most discussions for focus on sleeping bags, pads, etc:
Gear is important, but the big picture for some includes many factors, features and tips:
1. Fit: Down or synthetic a sleeping bag has to fit you - and the positions/movement you favor while in it.
2. Hood: that will snug down to a small hole around your mouth and nose
3. Layered sleep systems: (Lujon once posted something on this, but I didn't find it) work just like layered clothing - adaptable to the conditions
4. Layered sleep clothing
5. Pad: that protects you from cold spots
6. Sweat: Years ago in the scouts, we learned that a layer of moisture from the day's sweat can keep you from sleeping warm. Now, I don't understand this exactly, but it does seem I sleep warmer when I towel off (handi-wipes usually) before heading to the bag.
7. Hydration - might help too. I think of the body as a water-heated, water-cooled system which depends on enough circulating medium to work well.