We vacationed at WDW for Christmas 2 years ago. The parks seemed to begin getting crowded on Christmas Eve, and Christmas Day at MK was insane. We arrived at the park via WDW transportation just a little after 7am. The line for car drivers was already out of the gates. The park opened at 7am to park guests but was not crowded a bit. We had breakfast reservations at Cinderella's Royal Table at 8:30am and when we left, shortly after 9am, all we could see was a "sea of people", walking shoulder to shoulder.
I knew of this in advance and already planned for us to spend the day in Epcot where it was not very crowded. Since I planned for the crowds, I scheduled us to leave the day after Christmas as all parks would be extremely crowded.
IMO, it's nice to be at Disney for Christmas once but in the future I will schedule our trip during early to mid-December.