My wife and I were there during the last week of January this year, and it was moderately cold. Some days were warm enough for shorts and short sleeves, but the day we went to IOA we left our jackets at the hotel and ended up buying fleece jackets because it was just too cold. Not freezing, but cold enough that the fleece was necessary to enjoy the day. (It was the best $60 we spent all trip.)
It was also hard to find a good time to go swimming, as the pool was too cold if it was in the shade.
As for crowds, the lines were extremely short, never more than five minute wait for anything the whole week. In fact, on some roller coaster rides it was kind of a bummer, because a roller coaster goes faster when every seat is filled, but we had several rides where there were only a few people in our train. Of the crowds that are there, many are families with pre-school aged children so the "thrill"rides have no lines while the Pooh-type rides have short lines.
Best thing about going in January: most of the people you see staffing the parks are retirees instead of surly teenagers or college kids.