You can expect great and magical times. If you can try and go early Dec. The reason being, the closer you get to Christmas, the busier it will be and more expensive. Also, you will have
MVMCP = Mickey's Very Merry Christmas Party. You have the tree lighting at Epcot. That in it self is amazing. Hopefully they will have the Osborne lights at MGM this year. This past December was truly amazing. You get free hot coco and cookies at MK. They make it snow on Main Street. At MGM they also make it snow at the Osborne lights. AK is fun. Nothing like going to a water park in December. The weather can be strange from what I have been told. When we went, it was cold (sweat shirt weather) the first 2 days, but like in the 70's the rest. Make sure you resort hop. Nothing like Christmas decorations at Disney. The Grand Flaridian has a life size gingerbread house...AKL is amazing, but don't pass up the chance to visit the Wilderness Lodge.
Check out the Christmas threads. One tip, if you go the first week of December, that is when they film the Christmas parade that you see on tv, so it can get busy. That is done on the weekend.
One last tip...if you want MVMCP tickets, they usully go on sale May 1st...
Have fun and make memories....