I can't give any advice about your December dates, but this year, we went from May 21-29 and it was just lovely. No rain at all, and the crowds the first 5 days or so (Sat through Wed) were very manageable. Towards the end, as it neared Memorial Day weekend, the crowds were much more noticeable, but since we had been able to get all the "must-see's" out of the way those first few days, the crowds didn't bother us much. Weatherwise -- like I mentioned above, we experienced no rain (except for the first Sat night that we were there -- it rained for five minutes, which cleared out the parks, so by the time we got to MK, it had stopped raining AND it was nearly empty so we were able to walk on just about every ride in Fantasyland.) The temps every day were in the mid-80s. The first few days, it was extremely sunny, but towards the end, it was partly cloudy everyday, which made it more tolerable, and which was nice considering that it was more crowded in the parks.
Early to mid-May is a great time to visit WDW. We would definitely go during that time again!
Linda