Last year was the first time we traveled the week after Thanksgiving and we thought it was great. We were in MK on the Sunday after and thought the crowds were fine. The rest of the week just got better and better. It did rain on Tuesday for MVMCP but that didn't bother us. We rode tons of rides that night and saw most of the shows. By Wednesday and Thursday we were walking on everything.
It got a little more crowded on Friday, but by then we felt pretty spoiled by the crowds the previous days and took it in stride. We liked going this time of year so much that we're going again this year, but heading down on Thanksgiving day. We'll enjoy ourselves by relaxing the first three days and then hit the parks like we did last year starting on Sunday.
As everyone has mentioned, the weather is a total crap shoot. We had friends who were there the week of Thanksgiving last year and it was freezing to them, so you never know. We packed for everything and we were fine.
The week after Thanksgiving is a great time to go. The decorations are up, you're just a few weeks from Christmas, and everyone is in great spirits. If our trip this year is half as fun as last years then we'll figure we got our money's worth and then some.
Good luck!