Just wanted to emphasize that you really should be prepared for any kind of weather. A few years ago we were there the first week of December and this is what it was like:
Day 1 - rainy most of the morning and part of the afternoon, temps in the 70's.
Day 2-5 - skies were BEAUTIFUL, but it was COLD!!! In the 30's-40's. We even wore gloves. What worked best for us is layering, t-shirt underneath a sweatshirt or jacket, and jeans. When you get into the attractions, they are heated, so you'll need to take the sweatshirt off or it'll feel like a sauna. And BB will probably be closed on these cold days.
Day 6 - back up to the 80's. We went to BB, but the water was a bit cold. Day was beautiful, at night there was a thunderstorm.
Some people have been lucky and haven't experienced the cold weather, but I think it would be better to be prepared.
That said, this was absolutely my favorite time of the year!!! Definitely make time to check out the decorations at the resorts, and go to
MVMCP if you can. Also, EPCOT has lots of neat things. Make sure to check the calendars for all the events going on.