I LOVE early December!!! My favortie time to go!! We have been many times in early December.
As many have posted the weather is all over the place. We've had trips when it was HOT, trips when it was freezing, and trips with hot days and cold days all in the same week. This is the one bad thing about traveling during this time, you have to pack for everything. Even if the forcast says it's going to be hot pack for cold weather as well and vice-versa.
Crowds are lower than peek season but I have noticed a bit of crowd pick up over the last few years.
We love Mickey's Very Merry Christmas Party. I enjoy the atmosphere of the party. Ride lines are also usually very short at the parties. Sometimes we will do the party twice in a trip. Once to enjoy all that the party has and the other to just ride the rides as much as we want. My daughter's favorite ride is Peter Pan and "usually" at the MVMCP we can just walk right on over and over again. That sure beats a 30-60 minute wait during park hours.

So it's woth it for us to do that so that we can enjoy the rides to our hearts content.
Have fun!!!!