First, don't listen to people who are bound to tell you that you're crazy to visit during Christmas. It's wonderful.
If it's like past years, starting with Christmas Day, Disney gives it's guest a "gift." All of the parks are open early every day from Christmas through New Year's Eve for Resort guests. Again, this isn't something that's guaranteed, but they've done it in the past and there's a chance it will be done this year. If they do this, that's the time to get to the parks.
Bring layers of clothes. The weather can be lovely, but it can be frigid as well. We packed gloves, and ear warmers, and they came in handy on some days.
Make meal reservations. The restaurants will be booked solid and walk-ins are impossible. As was suggested, do the Candlelight Processional. It's best if you purchase the CP package that includes preferred seating and a meal at one of Disney's Epcot restaurants. In past years, you also got 15% off of Epcot purchases on the day that you have your ticket.
Be prepared for Holiday menus (i.e., higher prices). As I recall, it was just on Christmas Eve and Christmas Day.
Don't think that the MK will be too crowded on Christmas morning. We've been there twice on that day, and the park was empty. The crowds didn't arrive until early afternoon. We went to MGM once that happened, and the crowds were so light, we had no wait for TOT or RnR.
Since Disney is starting the 100th Birthday celebration, it's hard to predict what else might be happening.
You're going to have a great time!