We have been there 3 times in Dec and will be there the first week again this year. We absolutely love it there that time of year. Christmas is definitely in the air when your in WDW in Dec.
I agree with all of the mentioned posts above. See and do all thats possible pertaining to Christmas festivities. As your walking around, rushing to get to that next ride, slow down and take in all the little details. With 150 tractor-trailer loads of Christmas decorations, including 7 million lights, theres a lot to see. And theres a lot that can be missed. Granted you cant take it all in, but just pause every now and then to take in the small stuff.
One thing to add that hasnt been mentioned, ride the monorail over to GF and see the huge tree and the man size Gingerbread house. Its made out of real Gingerbread and it smells so good!!
Have Fun!!