I haven't been to WDW in Jan in quite sometime (see signature) but in my opinion is the best time to go.
Crowds--awesome. We did many rides over again without getting off and on line. It was nice to take pics down Main St. without 1,000 other people in the pics. We still made our reservations in advance but it would be much easier to get a walk up. We were also there during marathon times and it did not effect crowd levels. It was most pleasurable.
Weather--did get a bit chilly at night and on one day we were so cold we bought wool hats/gloves. Other than that one day jeans, t-shirts and maybe a hoodie occasionally. A nice am hot chocolate was always yummy on Main St., too. We still swam in the pools, too.
Decorations--one of my favorite parts of being there in Jan is that holiday decorations are still up. They look amazing!! Everywhere on property is decorated so beautifully. Two of the Jan times we were there we got to see all the docorations and then mid-week it all came down so we had it all decked and then normal...was cool.
Hours of Operation--the only real drawback is that the hours are shorter. You can still have EMH, so take advantage of that, but parks end earlier. This led us to Pleasure Island (no more

) and DTD each night. Check refurbs, too, as they tend to be more during this time.
It's a great time to go. I wish I could go back during that time!!
