We went the first week of Dec in 2002 and 2001 and the weather was very unpredictable. Freezing in the morning and in the evenings and nice and warmish during the day-even wore shorts a couple of days, but would have been comfortable in jeans.
We are also going in January of next year (trying to avoid the holiday crowds this time) and I plan on packing the same way I did for my December trips. We will be there Jan 25-31. I plan on doing laundry one day so I can pack less. This is generally what I pack for my 7 day winter trips per person and it has never let me down! (email me or pm me if you want the complete packing list):
3 tee shirts
3 long sleeve tees
2 pair of jeans
1 pair of athletic type pants or sweats
2 pair of shorts
1 sweatshirt
1 heavy type jacket (not down but water resistant nylon with sweatshirt type lining)
1 hat
1 pair of gloves ( we really have needed these at night!)
Our old rain ponchos from AK
socks/ underwear
For the kids (2 boys 5 and 7) I pack the above plus a couple of pair of long underwear that they can layer and take off/put on when needed along with extra socks and regular underwear.
I found on both of our last 2 trips we packed perfectly for the weather and did not have to purchase anything while there. Well actually we did have to buy DH a sweatshirt one morning when we went to AK for early entry. He left his sweatshirt in the room thinking it wasn't cold enough and he would be fine. After waiting 15 minutes in line for the park to open he was like an icicle and we had to buy him a sweatshirt (mine now!) for $40 ! What a fool!
The crowds should be as light or lighter than they were for our Dec trips, and the parks should be closing at about those times as well. This means all parks will close by 6 or 7 except for World Showcase which is open until 9:30. This works perfect for us since we so all the other parks during the days and save Epcot for the evenings! Illuminations will be on, but the fireworks at MK most likely will not be. We LOVE how it gets dark earlier-it really does make you want to buy a bunch of those glowy/light up thingys!
We never ended up doing the water parks, but we did do the hot tub at our resort (BCV) a couple of nights. It was nice and relaxing and only a little cold when running back to the room! Lots of people were swimming, but we are from AZ so we don't have that thick skin!! Luckily, since we don't do the water parks anyway during the winter trips, we just buy regular hoppers instead of plus hoppers or UPH and that saves us lots of $$ to put towards more character meals and souviners. We find for the amount of swimming we would do, the resort pool is just fine. Also, if you want to do a water park, I think only Blizzard Beach will be open in Janury of 2004 due to rehab of Typhoon Lagoon which starts the end of Dec. We just didn't see the point of an UPH if we couldn't visit everywhere with it.
Maybe we will see you there!