We have gone in December many times, and were in Disneyworld this past December from the 12th through the 20th. We love going this time of year for the Christmas decorations,
MVMCP is a must, and the Osborne Lights and the Candlelight Processional at Epcot was so beautiful.
One big word of advice, bring many different layers of clothes and a warm winter coat, hat, mittens etc. We are from Chicago so we are used to the cold, but boy, we sure didn't expect the COLD weather we got this past December in Disneyworld. We had 3 nights in the 30's and we FROZE our bottoms off. My 2 year old ended up getting sick with the flu and so did the rest of us. We had to extend our trip a few extra days since we were all to ill to fly. Not the best trip we have had, but we will probably consider going down to WDW in October since it is much warmer. The highs were only in the high 50's low 60's the entire week we were there. But the 30's was just awful at night. We didn't plan for the cold, so we didn't have very warm coats, gloves, or hats, so we had to buy what ever we could find. Didn't wear shorts at all, so if you are going in December just follow the weather up until you leave. Since Florida has so much humidity in the air, those 30's at night felt even colder in Florida than they do in Chicago, because our air is so dry. The dampness goes right through you! If you are prepared for cold, you will have a great time. I just wish we were smarter in our packing, so we didn't feel so cold all the time!
A few year ago, we did go the first week in December at it was in the high 60's low 70's a few days, but it does get cold at night!!!