We love to go in December. The crowds are lightest in the beginning of the month, and get more crowded as Chistmas approaches. From our experience, the crowds really start to roll in the Saturday before Christmas.
Also, be prepared for all temperatures. We have worn shorts and we have worn thermals. Be sure bring goves, etc from home.
MVMCP is fantastic. The lines are really managable. The fireworks are teriffic. The extra shows are all really good as well.
Epcot is magical, not only because of the Candelight Processional, but because each country tells its traditional holiday story.
MGM has the Osborn Lights, which are supposed to return this year.
If you can make time, try to take a look at some of the Christmas displays at the various resorts.
The Travel Channel did a special on Disney at Christmas last year. If you get a chance to see it, it will tell you all about everything that is going on at Christmas in Disney.
You will have a great time. In my opinion, there is no better time to go.