We've been at WDW twice in January.
It was quite cold most of the time we were there this past January and we were wearing our winter coats (although not the freezing-weather kind, only the it's-kinda-cold-out kind) nearly the whole stay. And, for us, it was definitely too cold to swim when we were there, but we did see plenty of other people swimming and enjoying it. We did swim in January '99, and the pool was lovely and heated . . . but we hadn't figured on what we were going to do when we got out of the pool!

Brrrrrrr!
But -- here's the great news -- during our most recent January trip, it only rained once and only for a little while late at night when it didn't affect any daytime park activities at all. So that's a definite plus on the weather. Also, after the Christmas crowd is gone (around January 3rd or 4th), there's no one in the parks and it's great walking on to every ride, not having to make PS for dinner, strolling around like they built the place just for you! We love it!
They take the Christmas decorations down bit by bit after New Year's, but, depending on when you're there, you may still see some, although by the 2nd week in January, I think they've pretty much disappeared. We were there Jan 2-12 this year and by the 10th, IIRC, the Xmas decs were all gone, but the first few days, the decs were still up everywhere. When we were there in '99, our stay started on Jan. 7 and the Xmas decs were completely gone by then...and so were the crowds! Also, fyi, our stay that year included MLK weekend and we really didn't notice any great upswing in the crowds, altho things may've changed since then.
Part of our stay this past year was during Marathon weekend, and other than seeing lots of people checking in to AKL, we didn't notice any more people in the parks.
It's great in January! You'll love it.
--miffy--