hands and feet
A few years back I purchased a pair of Lake winter bike shoes. Best investment I made in a while. If you are going to ride in the winter, these (or something like them) are a must, IMHO. Can't believe I spent years trying to get other bootie systems to work.
For the hands, I usually just wear a pair of windstopper fleece gloves, which is fine in all but the coldest conditions due to the sweat I work up while riding. However, just in case I carry a pair of goretex-type "overmitts" - basically a large mitten shell that goes over the gloves and goes halfway up to my elbow. If my hands start to get cold I slip on the overmitts and all is well. Since the overmitts are large and roomy, I can still works the brakes and shifters with them on. I find the mittens work better than gloves when the fingers start to ice up.
On the head I either use a fleece headband or a thin skullcap-type hat - dont find I need much there.
Lastly I always ride with a shell that has a hood - in the event I get really cold or have to walk out, I find the hood is a good thing to have in reserve.