...and although I did not go the parks on Thanksgiving, I was in the parks immediately after, and as others have said, the MK is crazy crowded! It is really just the most magical park during the holidays because of the icicle lighting on the castle, special Christmas light projections, and the way Main Street is decorated, not to mention other things like a cute holiday stage show in Tomorrowland. It is crowded, but so worth it. I remember telling DSis it was one of the best trips to the World we ever took because it was just extra magical. Honestly, we did not even book our FP+ reservations until we got there, and we just did them from our smartphones each day and really didn't have much of a problem. I found the wait times with FP+ to be very bearable, especially considering the weather is so mild. Some days were even cold (I actually missed the warm weather, it's kind of part of the experience after all!). I remember a few standby lines at MK were SUPER long though; Jungle Cruise ("Jingle" Cruise) must have had almost 2 hours. That being said, I was not bothered by the crowds, except for at Epcot in the World Showcase. People start lining up for the evening time Candlelight Processional literally from morning and the lines pretty much wrap around the whole WS. If you actually want to have a chance of attending this event (as in sitting in the bleachers) and don't want to waste a day of park touring waiting in line, try to get the dinner reservation package at WS, which guarantees you a spot and puts you in a different line.
Osborne Lights at DHS is VERY crowded. In general, if you are used to summer crowds, you should be fine...it might just be hard to get a good spot to watch parades.
Also, we had our Thanksgiving dinner at the food court of our Disney resort, and it was around $17 per meal. It was very crowded and they were even running out of certain side dishes. We also went to Downtown Disney on Thanksgiving, and it was not too bad as far as crowds.
Good luck!
