Like others have said, December in Florida can be totally unpredictable, so it is best to pack clothes for ALL weather just to be safe. We were there for 10 days last December and experienced cold and rainy weather for the first couple days, comfortable days in the high 60's and low 70's the next couple days, another cold snap, and our last couple days were in the mid-80's and we were sweating in short & t-shirts!
As for crowds, our experience was great! We did not stay onsite, so we avoided parks with EMH. We stayed pretty close to the recommended parks from TP and avoided the parks they said to avoid and experienced minimal wait times all over every park. Occasionally a line would build up, but we would just FP that ride and come back later. There seemed to be a lot of people in the park at times, but the wait times were still low.
As someone else mentioned, during this time of year you are contending with shorter park hours and limited showings of the nighttime shows. However, we found that with 7 parks days where we got there at rope drop or a little after and stayed till closing, we were able to do everything we wanted and then some. IllumiNations still runs every night, but Fantasmic will only have a few shows during the week, and you can only catch Spectro & Wishes on non-
MVMCP nights. We planned a night for each, and had no problems seeing everything we wanted at night. The Osborne Lights at DHS and the Candlelight Processional at Epcot are not to be missed!!