We were there December 13-20th this year. Crowds seemed moderate to me...not light, but then again, our prior trip had been at the end of August when the parks were dead. This year, they had MK opening at 8 a.m. everyday (not EMH) and that was wonderful, if you're willing to get up that early. I have pictures of my kids skipping though Liberty Square to Frontierland all alone (and that was after we'd ridden some rides in Fantasyland).
The weather was a bit of a shock for us. We had three prior October trips (plus one in June and one in August) and December was downright chilly in the mornings and at night, even for us, and we live in Minnesota. It just didn't feel like Florida...the kids didn't even want to swim. Maybe we got an especially cool week, but coats and hats and mittens were requirements at night.