I have gone to Disney World every year for at least a week from around the 16th to the 28th for the last 12 years. Just the two of us. I honestly think it is the VERY best time to go.
I am not a huge fan of children, and I feel that there are fewer during this time than any other. We eat at V&A at least twice. We do tours, and spa days. We get to really feel like adults for the holidays. In this world of toys and presents mainly for children, it is nice to do adult things in such a wonderfuly fun place. I do not have to go over to family events and bring presents to kids and listen to parents boast about children. You honestly will not miss spending time with the family, you can do that at New Years or at Thanksgiving. Children can be a joy, but too many times arguments stem from "parenting" issues. It is not worth the drama. I just give to the local toy drive. At least some children that won't get anything will.
The decorations are amazing and romantic. There are good deals on hotel rates and it is fun to come back and take a nap in the afternoon.
I feel that we get better service because the waiter does not feel compelled to entertain us the way they do with parties that have children. We get served fast and well and they are always very nice. Sometimes parties of two will be seated in better areas. California Grill has an adult only room. Coral Reef seats the doubles right next to the glass. Brown Derby seats your in the booths.
Sometimes because you are not stressed out with children at check in and are polite you will get a room upgrade. I have never not been upgraded at any hotel. Each hotel front desk welcomes a friendly nice face that offers a smile and does not complain.
Please do consider WDW for Christmas. I think it is well worth the trip and the memories will be countless.