We went for our first trip to WDW last Thanksgiving Week. We arrived on the Saturday before and left the Saturday after. Since it was my first trip, I can't compare it to other times of year, but we have been at DL on Thanksgiving, and this was somewhat lighter. We did a commando raid on Magic Kingdome Thursday morning, getting the major attractions in Frontierland, Adventurland, and Liberty Square under our belts in less than 2 hours, then headed to Epcot for the rest of the day. I think that helped us miss the worst of the crowds.
We did Studios on Friday and with some aggressive Fast Passing, did Rock'n'Roller Coaster and Tower of Terror about six times each, then coasted through shows the rest of the day. Then Animal Kingdom on Saturday morning and early afternoon, and home.
None of the parks other than MK seemed crowded, or at least unmanageably so, and being there for the Christmas decorations and events is always wonderful. I think the days leading up to Thanksgiving were great - generally small crowds, minimal waits. It was warm and not humid after the first day or so -- really, an ideal time to be there in my opinion.