We visited from England for two weeks, with thanksgiving week in the middle in 2002. MK was 5x more crowded on thanksgiving Monday than the week before (where we walked on most rides all day, and had easy access to characters). MGM was bursting on the Wednesday but we got on all the rides, saw Fantasmic (arrived 30 mins b4 start time) and enjoyed the Osborne lights. We spent Thanksgiving day at Epcot which didn't seem affected and didnt experience a wait at the dining reservation we had made some 60 days in advance (San Angel Inn).
Can't comment on the Friday thru Sunday - we visited IOA, SW and US which never seem to be as busy as WDW.
If your aims is to ride all the popular rides, my advice is to get to the parks at opening times and ride the busiest rides first off. Use FP for as much of the rest as u can. If you turn up much after 10:30 you'll miss out on some of the popular rides.
Having said that, I have no doubts you will have an enjoyable time regardless of the crowds. Its not all about rides - the parades, fireworks and general ambience of the parks are always guaranteed to create a warm fuzzy feeling.
Also the weather is great - dry and cool. Perfect for visiting parks.
The few days after thanksgiving were quieter, but still the Movies resort hotel was noticeably busier than the week before thanksgiving.
Overall, as we were able to visit MK, EP, MGM several times, Thanksgiving didn't impact on our trip too much. Having said that I won't visit then again. The holiday doesn't mean anything to us being from the UK, and there are other weeks either side that are less busy.
Our previous trip in 2000 was the 2 weeks after thanksgiving week and was much less crowded.
This year, because of school issues we are visiting from Oct 17 to Nov 7, (two weeks of which are school half term holidays). They recently changed the school term here in the UK and reduced the summer to 5 weeks and increased Autumn to 2 weeks).
I'm Hoping the crowds will be similar to when we first visited. Fingers (and everything else crossed!). But I think we will miss the Christmas theming.
You will definately enjoy.