Capris one year and sweat suits the next. I have almost frozen to death in Disney during the first week of Dec. It has been several years...but I am pooh sized and a human furnance....so it was memorable.
DH and I always check the 10 day forecast before packing. Then 3-4 days out we actually start filling the suitcases.
Layers are always good. T-shirts, maybe a long sleeve shirt and thin jacket. No need to ever wear a ski parka. But it is cool sometimes moving from noon heat into airconditioning.
All depends where you're from and what your tolerance is. We've been at WDW for the 1st week 3 times (2000, 2005 and 2008) and each time we wore t short sleeves and shorts all the time, day and night and were comfortable.
We're from northern Wisconsin so coming from an avg of high's of10- 20 degrees going to 75-85 degrees was a bit of a hot change for us. And yes we got looks from people with sweatshirts and jackets on when it went down to 60's at night.