We went November 2002 from the 16th - 23rd (week before Thanksgiving), and LOVED it !
Not only was it not crowded when we went (which is what others told us before we booked it), but the weather was beautiful, AND each day we were suprised with new Christmas decorations appearing throughout our resort and Disney World by the elves while we slept.
That's right parks were not crowded, and the MOST we waited for any attractionwas maybe 15 minutes (includes Test Track, Rock'n Roll Roller Coaster, etc).
The weather was excellent considering the time of year, and reached the low 80's high 70's during our stay, and we were able to do Blizzard Beach during our stay. A couple of days it was a little unseasonably cool, and we just put on a sweatshirts to go to the park, but no big deal. At night you could wear shorts with a sweatshirt.
As far as the decorations, you get to see new decorations and the Disney characters dressed up in Christmas costumes throughout the resorts and parks.
I definitely recommend the week before Thanksgiving.