I don't know where you are from, but I'm in Mass and when we fly to WDW in the late fall/early winter, we fly in jeans, turtleneck, sweatshirt, windbreaker, heavyish socks. Then, we can take off layers on the plane. This way, we have our 'warmer' clothing, and don't have to take up space in our luggage. We do pack shorts/short sleeve teeshirt in our carry-on bags tho. We have made the mistake of having to walk around the parks in our jeans and longsleeved shirts when it's been in the low 80's because we didn't think to get into our stored bags.
We have found that you can get away with shorts with either a short or long sleeved shirt during the day. Jeans with a sweatshirt are perfect at night. In '03 we were there in mid-Dec and it was downright cold!!! We did wear our mittens on our last day that year!!! But, that was a bit unusual from what I understand.