We went mid Jan 05. I'd brought long sleeve t-shirts for everyone, plus sweatshirts, plus windbreakers with stretch mittens (the kind for $1 that you can stuff in your pocket) and headbands (again thin stretchy ones that primarily kept your ears a little warm).
At the last minute, I decided to throw in our long sleeve hooded polartecs, since we were staying 4 days, I thought then 2 days we could wear our sweatshirts and 2 days our polartecs. My husband thought it was silly to pack both for such a short trip. Anyways...we ended up wearing our LS shirts, under our sweatshirts, under our polartecs, under our windbreakers with our mittens and headbands most of the trip - particularly in the morning and evening EMH's. Mid afternoon it got warm enough to take off one layer - that's about it - highs were in the 50's but we have pics of us on Splash Mountain at 9:30 at night when it was sleeting/raining and obviously much cooler.
However, my folks went two weeks after we got back (last weekend in Jan) and it was near 80 each day. They wore shorts and t-shirts with a light jacket and were fine. I'd definitely watch the weather forecast and as a minimum have something warm and hooded - whether a lined jacked, polartec or "hoodie", particularly if you have kids.