We were there last Jan. 9-19. It was during the worst cold snap in 50 years. High temp was 34 the day we arrived, and there were snow flurries!
That said, we had a blast! There were no crowds (except for MLK weekend). We're going again next Jan, but after MLK weekend.
Everybody tells you to pack for all kinds of weather. We are definitely going to be watching the weather, but I plan to bring more cold weather stuff than warm weather stuff. My ds lost his gloves the first day and we could not find any gloves ANYWHERE in Orlando. I figure we can always buy T shirts and shorts if we need them, but finding thermals in Florida may be tough!
The weather did warm up over the time we were there. By the end of the week it was in the 70's and we were wearing shorts. We always brought sweatshirts, because it was in the 40's-50's in the am, even on the warmer days. And it does get COLD in the evenings.
Over MLK weekend the crowds were much, much worse than midweek. We were walking shoulder-to-shoulder with people at HS on Sunday (Fantasmic night). We were at HS on Friday before MLK and there might have been 20 other people on the whole block at Streets of America. As long as you avoid MLK weekend you should have very low crowds.