We went to Disney, Dec 1-10th, 2001. We had unseasonably warm weather the entire time we were there. It was so warm we didn't have to wear a coat at night. It was plenty warm to swim either day or night. But like I said, that was not seasonal weather for that time of year. We lucked out!
As a complete surprise to us, the lines for rides were none, although on the weekend days it was much busier. Because there was no wait, the parks hardly used the fast pass system. The Christmas decorations were already up during our visit, and also some of the characters were dressed up in holiday attire, which made for great holiday pictures. The only thing that stinks is that the parks do close early for the Very Merry Christmas party and all the other events going on. That didn't bother us because walking around for 6-8 hrs was plenty for us with little kids in tow.
The day we were at the Magic Kingdom, they were taping the "Very Merry Christmas" parade which they show on Christmas day. We sat down and watched the parade, and they go around taping the crowd. My children were taped quite a few times, and as we watched it on Christmas day, we were surprised to see they got on there, not only once, but twice! That was the best memory of all.
If I were to go again, I would definately go the first week of December. Thanks to all info on this board, it helped us decide when to go.