I was hesitant to reply because this really depends on the person, speed, the commute, and the weather. Alienator and others have given a lot of info and I am only adding my own specific approach that may or may not work for you. If you ride at a fast speed you may start out cold but warm up pretty quickly. I rode all last winter on a 7.5 mile (each way) commute that is mostly packed gravel. I road in the rain, cold, snow. I'm an avid skier and am used to being active on cold mountains in the winter so I may be more acclimated than some. I find riding on cold days to be very exhilarating.
On coldish rainy days, I wore a GoreTex skiing shell, with GoreTex pants that cover shorts or cycling tights. I also have a rainproof cover for my helmet and waterproof insulated winter biking shoes. On really cold days, I first started out with a heavier insulated skiing jacket and found myself sweating too much because I ride pretty quickly. I finally settled on wearing a long sleeve polypropylene undershirt, covered by a long sleeve biking shirt, and the same skiing shell. I wear a headband which keeps my ears warm and also have used a Balaclava a few times on really bitterly cold and windy days. I generally wear just winter biking tights on my legs but occasionally I have covered them with a pair of nylon sports pants. I wear thick wool socks and use my skiing gloves. I really don't have too much problem with my feet getting cold on this commute so I can do it with MTB shoes and neoprene covers. But on very cold days on longer rides my toes will get cold and the winter cycling shoes help. I think if your legs are cold you could try a pair of polypropylene long underwear over biking shorts followed by heavyweight winter biking tights. (I prefer biking tights without a chamois and wear shorts under them.)
I do agree with Alienator that the best gear (especially tights) is probably the stuff designed for cycling. If you don't want to buy biking tights you could use long underwear and some kind of athletic pants over them if you don't mind the extra material flapping and strap the leg off from getting caught in the chain. You could probably improvise on some other things too. While I do sometimes use a Gore Windstopper (non-GoreTex model) biking jacket - which is excellent and generally warm enough when layered, I like having the pockets and hood from my skiing shell which is also bit warmer and dryer. With a little trial and error you can get it down so you will be comfortable enough and arrive at work without being sweaty.
I have very good lights as the trail I ride is pitch dark after 5 in the winter and also carry a small extra light that I can use as either an emergency headlight if the good one dies or as a flashlight if I need it. I also bring along a sweater or jacket liner in case I break down and have to walk a distance or fix something.