When my family (mom, dad, 2 sisters, aunt, uncle, 2 cousins) went to WDW in 2003 over Thanksgiving, we did The Brown Derby for lunch. 4 of us had the traditional turkey dinner--turkey, stuffing, the whole nine yards...it was absolutely delicious! I would definitely go there if you want a yummy dinner! The upside of going here is that there are several other different things on the menu, if perhaps you don't want the main meal--my Dad had a Cobb salad instead. On top of that, MGM isn't quite as crowded as the MK on Thanksgiving Day.
Liberty Tree Tavern has the traditional Thanksgiving Day food. That's if you will be in the MK that day. We are going to LeCellier for Turkey Day because we will be at EPCOT.