Typically, base layers are to keep you warm but they also should keep you dry through a wicking action. REI's site talks about different fabrics for base layers and how they are best used:
http://www.rei.com/expertadvice/articles/underwear.html
I have a problem with using a layer under a jersey because if you warm up it is hard to ditch the layer. I guess that is why people get arm warmers. So far, when it is cold I wear a wool top with a partial zip and a light jacket. I have a rain jacket that isn't vented much and a wind jacket that is well vented.
Today it is cold down here and my cousin and I are going for a short, maybe 6 mile ride. I am going to wear jeans and long underwear because I am a wimp.
