wow, a lot of questions...you may want to do a search with the word thanksgiving.
1). The weather can be variable...sometimes very warm, sometimes rain.
2). the parks are crowded, but I didn't think they were too bad considering it was a major holiday. I've seen worse in the summer. The only place we saw crying children was at MK... and that I blame the parents, not the children!!!
3). you can check the park hours on the main page or go to Disneyworld.com, they have them all listed... usually MK is opened till 10pm, Epcot (FW until 7) WS until 9pm or 9:30, AK closes early usually 6pm, but a few nights they'll be open until 8pm, MGM usually 8 or 8:30
http://disneyworld.disney.go.com/wd...KCalendarParkHoursMonthlyViewPage&date=112004
4). decorating starts at the end of October/beginning of November and is usually done by Thanksgiving week.
5). prices are expensive, usually no discounts unless you're buying a package...sometimes you luck out and can get a code.