The week after Thanksgiving is a favorite of ours - we usually go the first week of December - this year it's 12/1. The crowds are generally low and the weather is usually great. It can get cool at night, so bring a sweater.
The nicer hotels have great decorations - the HUGE gingerbread house at the Grand Floridian is a must-see, and the pastry chef at the Beach Club always has something grand, like a carousel.
All the parks are decorated beautifully - there are trees made out of masses of pointsettias and ribbons everywhere. Mickey's Christmas party is also on at this time.