Hi Disney-Kim,
Yes, it will be crowded, but there are ways to deal with that as jlima said. The early entry is the way to go. Use that time to hit the high priority rides and get fast passes for some of the others. The Plaza Inn serves a nice thanksgiving dinner, good food, reasonable prices. Unlike some of the other excellent restaurants that others have mentioned, this is a buffet style place. Its still a nice compromise of cost vs dining experience.
Not all annual passes are blocked out though. Premium Annual Pass holders have no black out dates at all: or "365 days of fun". We plan on being there!!! Its a blast! Don't be worried about the possibility of crowds. Just go, be relaxed, and enjoy! Sure, some people get stressed and rude, but sometimes being polite, cheerful and courteous in return causes them to catch that couteous attitude and you actually see them smile. It contagious, but in a good way.
Be on watch for the "ebb and flow" of crowds at the park during the day to take advantage of short lines. For example, during the parades, people are drawn away from other attractions making lines shorter. Also, it can be a good idea to do everything in a particular "land" before moving on. You save a lot of walking time that way. Those 10-20 minute walks really have a way of adding up! Add Thanksgiving day crowds, and and a rushed attitude and it can become quite the ordeal. This is when planning really pays off.
We usually visit
disneyland/DCA/DTD about 25 to 30 times a year. Its so much fun to see the place change during the various seasons of the year. Well, as much as seasons do change in southern california!
Darian