December is one of those months I wouldn't worry about over packing. You will most likely be glad you did. When I was there in 2010 we got quite the cold snap for a couple of nights. I was wearing literally everything I had, a t-shirt, long sleeve shirt, and two sweatshirts and I was freezing! I am from Maine too by the way and no stranger to cold! My Mom and I had to buy hats and mittens from the Emporium, talk about expensive. BUT on the other hand we also had most of the Magic Kingdom to ourselves that night, come prepared and you could have some quiet nights to yourself. Just remember at night you won't be running from ride to ride, at least we don't, but talking in the fireworks, MSEP, Osborne lights, CP, etc, dress accordingly so it doesn't put a damper on your fun!
I will also say that our first December visit in 2001 we got quite spoiled and did a waterpark 2 afternoons of that trip. It was in the low 80's and beautiful. We were some shocked when we went back the next year and it was freezing