A few comments, rather random-
We went once for Thanksgiving, mainly to see what it was like.
There were no special Thanksgiving decorations like at Christmas.
On the day after Thanksgiving we were the first to see the Christmas tree in the MK when we entered; they, at that time, put it up on that day.
Christmas decorations in general were in place on the Friday after Thanksgiving.
You might want to reserve a place for Thanksgiving dinner or lunch. A lot of the eating places have special menus for Thanksgiving. We went to Mickey's Backyard BBQ which had a special Thanksgiving theme for that day. It was very good. Got stuffed- all you can eat.
The crowds were not bad, except on Thanksgiving afternoon when it seemed that all of Orlando had finished their Thanksgiving meal and then headed for the MK. The Friday after Thanksgiving was more crowded than the days before it.
The airlines stick it to you, price wise, because everyone flies at about the same time for Thanksgiving. Reservations are harder to get than in December.
We've decided to go back to visiting WDW in December.
Actually we'll go anytime we can, though.
Have fun.