We were there over Thanksgiving week last year. It was pretty crowded, but we were to thrilled to be there (our first trip to WDW as a family) that it didn't matter how many rides we got on. This year, we're going at the same time, but using a touring plan to maximize our activities... we'll see how that goes!
Many days, it was warm enough for t-shirts and shorts, but the mornings and evenings were jeans and sweatshirt weather. As always, if you're leaving the hotel for the entire day, layer a t-shirt under something warmer and keep the extra clothes in a locker (very hany and inexpensive) until the sun goes down.
When we were there, there were definitely decorations on Main Street and the hotels. We're going to
MVMCP as well, so that will be full of Christmas cheer! It also means that decorations should be up by the 17th at the latest. There may even be another Christmas Party date before ours.
There was only one thing we didn't enjoy 100% and that was Thanksgiving dinner at 'Ohana -- the food was mediocre and we weren't too in love with the family-style servings. I tried to get LTT for this year, but they're booked solid. We've planned dinner at Planet Hollywood (we all decided that having turkey and trimmings isn't a priority for us) and going to Cirque du Soleil afterwards. I hope that makes for a memorable Thanksgiving!
Enjoy your trip, we'll be there at the same time, so perhaps we'll run into each other!