We were there Nov 22-29, 2013--the Friday before to the Friday after Thanksgiving. The only
MVMCP during our trip was on our arrival night, the 22nd. We went and my girls (ages 9, 11) had fun but we didn't get full enjoyment from it because we had been up since 4am for our 6am flight. Wouldn't do it again on arrival day.
We enjoyed low crowds on the first weekend but by Monday, yes, the crowds arrived. Not Christmas crowds but more than July crowds. Between the crowds and the FP+ rollout (we were offsite so had no access) and the very limited supply of legacy FP, we basically toured only at ropedrop for a few hours and when it got crowded we left. We did the Osborne Lights one night and loved it, but other than that we ended up spending only 4-5 hours a day in the parks. We did spend a day at Sea World and honestly we totally enjoyed it. No crowds, no lines, we saw all the shows and rode all the rides we wanted with no long waits and no need for a FP! We stayed all day and enjoyed it immensely.
The weather was another issue---we had temps from the 30's to the 80's during our week. We wore shorts once, no swimming. On Thanksgiving morning it was in the 30's at 8am rope drop for AK. We made a Target run for extra sweatshirts and were wearing our Northface jackets and gloves! It was chilly. It was also in the 40's the night we did the Osborne lights. Only one brief downpour for the week though. You do have to pack for everything that time of year.
All in all---we were glad we went and got to see the Christmas decorations. It was worth it for that. The FP+ rollout was a major downer on our actual touring and made it much harder for us to manage the crowds that were there. For this reason we aren't looking to book a return trip any time soon (and we have been yearly visitors for a number of years) so I am glad we did our "Christmas" trip.