I pretty much wear the same mountain biking as road biking. My legs are usually the cold spot. I wear a pair of Sporthill windproof pants with long underwear underneath. However if it's much below freezing I wear my Mountain Hardwear tech windstopper pants. They are great at keeping me warm, but are kind of constrictive around the knees while pedaling. No need for a chamois.
On top I layer as many base and mid layers as needed for the conditions and use a windstopper shell. A windstopper skull cap works well under my helmet. I usually carry some chemical hand warmers to put into my gloves as needed.
Most of my winter biking clothing is not bike specific, just whatever is needed to keep warm.