I've been in Dec 2 times, Early Jan once and Nov 2 times. It can be very HOT (90's one Dec trip) and very COLD (30's in Jan & 1 Dec trip). I wear jeans on the plane and take another pair along with long & short sleeve shirts and shorts. I also bring a denim shirt, fleece and rain jacket which can be layered.
On my Jan trip I was with DH who doesn't travel especially since he was nervous as this was our first family trip flying, my DD(4 1/2), DS(turned 3 on the trip) and DD(11 months). We had a double stroller and 3 car seats. I had just 2 checked bags and 2 carryones (plus stroller & car seat). To keep the luggage down I mailed a box of diapers, wipes, and EXTRA clothes to the resort.
December is a great time to visit the World-I really love the Santas around the world at Epcot. I'm sure you'll have a great trip!
jwfla422-I just got home late Wednesday night too and I'm from Boston and I didn't think it was too bad. I was only there for 3 nights but I pretty much thought the heat was more bearable than I anticipated. And the parks were empty! We ate at CP one morning at 8:10 and hit Fantasy Land just after 9 and were able to do Peter Pan, Small World, Carosel, Dumbo, Pooh, Snow White, Tomorrow Land Speedway (I HATE that thing but I'm such a good Mom I had to let my DS ride it when he asked for the car ride. This was a trip for just him I left his sisters at home. After we got off I asked him if he liked and he said "Yeh, but I want to go on the cars that drive themselves" [Test Track-which I love]), all in less than 1 1/2 hours. We only waited at the speedway-even on Dumbo we were on the next "flight". The only thing I got a fast pass for the whole trip was Buzz once (rode it at least 4 times with little wait the other times) and Soarin (which was so COOL!!!). I honestly didn't think it was that much hotter than it has been in October and the crowds were much smaller. I would absolutely go the last week in August again but December does have a real magic to it.