Hi,
Depends on when, as stated w/e's busier. We went the 21 - 23 Dec last year and it was as busy as the summer although in saying that, depends on your preception of busy! Be prepared for queues I guess.
As to the weather, it was dry and cold and there was thick ice on the small lakes around Disney - you can wrap up against the cold but if it's raining then that is different - pot luck really!
Rgds
A