The Mountain Hut
When hiking between freezing and near 20 degrees, its ok to use just shells and thermals(underwear). Otherwise you may overheat. If its colder, you can throw on an insulation layer underneath your pants shell, which should be equivalent to either polar-tech 200(mid-weight), or polartech300(expedition weight). They are just like a real thick pair of sweat pants that slip over your thermals. Always use synthetic materials here. If your in subzero conditions during daylight hours, then you may want to look into some down pants, or down bib/overalls to go over your synthetics(thermals and maybe even insulators).
Pant shells are imperative. They need to be made of good quality, and of rip-proof material. These are your pants, while hiking, and outdoors. They keep the wet off, they keep the wind off, they keep the heat in, they keep the dirt out, and they keep the sweat moving out, and even sometimes take food spills. Black is always a good color, so the sun can help heat it when possible. Another thing that's almost a must have, is side zippers. Either full or partial is ok. Full side leg zips have their problems if they break, but are sure nice to open up all the way when the body core streams heat out.
When in camp its good to use the expedition weight insulators. I always place these over my thermals, and hang out in camp like that frequently, unless its storming. once in the tent, this is almost always the standard wear, and shells are never needed. when sleeping think twice about using the insulators, thermals are plenty warm in a good 0 deg. mummy bag, unless its well below 0F on the outside of the tent(inc.wind chill).