We cruised over Christmas in 2010, so things can definitely be different now. Christmas was a sea day, which was perfect for our family (me, DW, DD14 and DD12, at the time). Santa was in the Atrium Christmas morning. As others mentioned, it had a line, which our DD's got in. DW got in line for the buffet in one of the MDR, as it also had a wait. I bounced between the two lines. Shortly after the girls got their picture with Santa, we were able to be seated for brunch. I think they also had hot cocoa and cookies in the atrium Christmas morning.
There was a midnight Mass Christmas Eve in the Walt Disney Theatre. This was open to all guests and crew. It was a joy to be worshiping with a variety of people and nationalities. We looked around just before the service started, and saw a number of crew, from Entertainment Staff, Maintenance Crew, Servers, etc. It was neat seeing them all in street clothes, mostly dressed more nicely than the guests. My guess is many do not have the opportunity to dress like that often on the ship. And my DD's got to pass the peace with Prince Charming's friend.
One thing that was odd (not bad, just odd) was the normal evening entertainment Christmas Eve. I think it was a hypnotist. I don't know what they should have done different, but it was weird watching him Christmas Eve. DW and I also went to the adult entertainment that night after seeing the main show. Going to the Midnight Mass later that evening helped me normalize the holiday again.
I highly recommend a cruise over Christmas. We have been sharing memories of the cruise this Christmas.