I have gone to WDW many times the weeks after Thanksgiving. There is a "low" period between the week after Thanksgiving thru the 15-16th of December from a crowd perspective. After the 16th or so, the crowds ramp up to capacity for Christmas/New Years.
The Christmas decorations should be up either way...I believe they transition from Halloween to Christmas sometime in November.
The weather should be beautiful that time of year. However, do not be suprised if the lows are in the 40s and the highs in the upper 50s to mid 60s. By the almanac, it should be in the low 70s but it never seems to work out that way.
Personally, I think the best time to go around this period is the 2nd week of December. My wife and I usually schedule a trip roughly for Dec. 6-Dec13. Good weather (usually), moderate crowds, park hours are not truncated too badly like in other off-peak times and since we plan several months in advance, we always get outstanding deals on airline/hotel rooms (we stay offsite with Marriott).
It's a great time to go...my second favorite time slot behind Mid-October. You will enjoy Disney that time of year.