I agree with Malcon10t - it could go either way. Sometimes it is very warm for December, and other times it is quite chilly and rainy.
I switch my December trip dates around. There are times I've gone on the weekend right before Christmas. Two years ago I went earlier than that. Last year I was there from December 4 - December 8. One of the days (December 5th, I think it was) was incredibly windy - like windy to the point where my ponytail was standing straight up on my head a couple of times. Is it likely that there will be a day exactly like that again during the early part of December? Probably not.
There are other December DLR trips I have made where it was in the 40's at night and a coat was necessary, and other times where it's been in the 50's at night and I was wearing a t-shirt the whole time, with no coat.
In any case, you'll have a great time! It's the best time frame to go (in my opinion) - generally, it seems that a lot of people have gone back to work and school after Thanksgiving break and have not yet gotten out for Christmas break. So that early December slot is a small window of opportunity for slightly lower crowds, while still being able to experience the full holiday offerings around the Resort (including the 3 Disney hotels).
