Layers. And possibly cold medicine. (Sadly, I needed it last December, but I didn't let it get me down. If I had to be sick, at least I was still at WDW!)
Temps are very difficult to predict, so you should pack some of everything, clothes-wise. Layers, layers, layers! We have a lot less rain in December than in the summer, so packing a poncho or two for each day would be excessive.
Otherwise, packing for December isn't different than July. Toothbrush, toothpaste, etc.
Gum. If you chew gum, you cannot buy it on the WDW property or at the airport.
Register on WDWMAGIC. This sidebar will go away, and you'll see fewer ads.