The last time we went in early December was 2004 and we had moderate winter weather for most of the trip (jeans, long sleeve shirts). We thought about swimming but the cold walk back from the pool to our room was enough to make us decide to nap instead of swim for our relaxation.
I do advise you bring at least a pair of gloves (a hat and scarf would be even better). We had one night at Epcot where we were miserable, sitting on the cold benches for the Candlelight Processional and then fighting the wind and chill while walking around. In the end, my mom got an ear infection and had difficulty on the airplane ride back home.
We had plenty of layers but simply forgot to bring something for our hands, neck and head. An extra shirt or small baby blanket would have been nice to sit on instead of the cold metal benches.
Also, if you are in Epcot, get your coffee or hot chocolate prior to the Candlelight Processional letting out. Otherwise, the lines are incredibly long and they run out (yes, after standing in line we were told they ran out of hot chocolate mix!)
Have a great time!
Marsha