We were there Thanksgiving week last year and at the MK the day it rained all day...it was more of a misty/cold/drizzly rain. I think it must have deterred some people from the park because we were there at rope drop and before 11 a.m. (8 a.m. opening) had ridden every ride in Fantasyland and Tomorrowland with the exception of SDMT (we had a fast pass for later in the day)! We also met a number of characters in that time frame (Alice, White Rabbit, Stitch, Buzz Lightyear, Rapunzel, and Cinderella). I say bring on the rain! It was also about 55 degrees, which may be cold by Florida standards, but it was perfect fall football weather in Pennsylvania!
Overall, we went to the most recommended park (EasyWDW) and did rope drop a few times, utilized fastpass+ and didn't notice that the crowds were bad at all. The only thing we waited a long time for was Rock'n Roller Coaster at DHS because we chose to use our Tier 1 fastpass for Toy Story Mania. We've been twice during Thanksgiving week and loved it! The only real bummer that week was that we attended
MVMCP on Tuesday, December 2nd...decided to wait for the 2nd parade to beat the crowds a bit and it DOWNPOURED (like fill your shoes with water downpoured) and they cancelled the 2nd parade. We stuck around for the rain to stop and most people had left so we met the Seven Dwarfs with a 15 minute wait and walked on to SDMT right before midnight.