ALL the resorts and parks have some form of holiday decorations.
This will be our fourth year in a row at WDW for the week of Thanksgiving and the following week. The week of Thanksgiving is always a madhouse. There's no way we can get around that. But, the kids have the whole week off from school and we don't want to be home twiddling our fingers. So we tour the parks knowing if something is too busy, we can always do it next week. Personally, I like just "being" at WDW.
We've found the week following Thanksgiving to be truly magical. The holiday crowds are gone, we attend a half-empty MK during
MVMCP,
ALL the holiday decorations are up and the holiday festivities begin within the parks. Even with our little secret out on this being "the best" time to visit WDW, it is still a great time to go. We don't miss it!
Now I will tell you, some people's opinion on crowd levels totally varies. I was reading Trip Reports from the same time we were at WDW in December and they were complaining how busy it was and blah blah blah. We never encountered any of that. Having some form of plan in place, no matter when you visit WDW helps too. Even on the slowest day of the year, there's still going to be at least a 60 minutes stand-by wait on TSM and Soarin'. Go figure. But, that's the reality of WDW and that's what FPs and quick husbands are for. DH always goes ahead of us to get our TSM and Soarin' FPs.
We just got back from a hot, humid and crowded WDW. Never again will we visit WDW in the summertime. We love WDW in the Fall/Winter. If it ain't broke, don't fix it.

Jennifer