After a winter of riding through 2 polar vortexes and many days of sub-freezing temperatures, I suppose I have some experience at properly dressing for severe cold.
I'll start from the ground up with what I wear when riding in cold temps:
Boots: Land's End Extreme Squall boots
Socks: Smartwool (two layers under 20 degrees)
Pants: WOOL! I like Ibex woolies and tights in 150, 220 and 370 weight depending on the temperature.
Top: WOOL! An Ibex hoodie is great for not only a layer, but the hood keeps one's ears warm too.
Hands: above 32 degrees: a liner and REI Stratos waterproof lobster gloves. Below 32 degrees: liners and Cabela's Gore Tex Mitts.
Neck: Fleece gaiter I'm thinking of switching to a wool gaiter. I'm not fond of how wet the fleece stays from my breath.
Eyes: Above 30 degrees, a cheap pair of safety glasses, which are also great at keeping river trail-loving swarms of gnats out of my eyes in the summer. Below 30 degrees: a pair of Giro ski goggles.
Head: my Bern helmet is lined and ideal for warmth, I may have to find a new, more vented helmet for summer. A fleece ear band keeps my ears protected.
Extended periods riding in 20 degrees or lower require hand and toe warmers. My brand of choice is Grabber.