For winter riding, I usually wear sweatpants over some baggy MTB shorts with a padded bottom. That is pretty good to a little below freezing. Colder than that, I add some Pearl Izumi leg warmers. Those keep me pretty warm, except for the small gap they leave between them and the top of my socks. The other problems with the leg warmers is they can creep down after a while, rendering them useless.
So I'm looking for a warmer solution for my legs. The problem is I am not a huge fan of tights. I bike commute to work... personally, I'd feel funny showing up with my junk on display. I think the main problem with sweatpants is the wind goes right through them, so I'm thinking something similar to them with some wind-proofing would be good.