Decorations in WDW are second to none, whether the parks or the resorts, they are worth seeing. We've been to WDW first week of December a couple of times a bunch of years ago and just returned from a first January 2008 trip. Here's our take, we're from the northeastern part of Ohio and really hate the cold weather and mainly go to Florida for the hot heat and high humidity. The times we were in December were on the chilly side but very nice for touring. Past January was just right, we hit it perfect, weather in 80's and got cold when we were leaving. Crowds either times were manageable, but I think the secret of slack times is out now so no way to judge crowds at so called slow times, its like the weather, luck of the draw.
All in all, I really prefer the October period before Halloween, lesser crowds, again luck of the draw, still the enjoyable heat of Florida, we spend alot of afternoons in and around the pool. Early November is good too before Thanksgiving, decorations for Xmas are up and you get the best of both worlds, decorations and less people. That being said and through these forums, slack times are filling up along with the free dining, WDw just seems to be getting busier on our secret times. Doesn't really matter, there's enough of WDW to go around, just hit the parks early, spend the noon hours at your resort enjoying the amenities and pool and go back to the parks in the evening, thats what we do no matter what month.