What you can see at Christmas time is good -- decorations, Osborne lights, some shows and parade change for Christmas themes; atmosphere is very Christmas. Weather is usually OK but can vary significantly -- your "average" high is low 70s but it is the time of year, unlike May or October, when "average" is not usually what you get. Can get cool (highs in low 50s) days, rainy days (not just showers but all day), very warm and sunny days (80s) and all in the same week, and sometimes in the same day.
The crowds are enormous. If you arrive the 19th it will be crowded but not yet complete but will build quickly for a few days and then hit full force. Take your May crowd, add the total guests from then to your total Oct crowd, and then multiply by two and you will have Christmas. You cannot go expecting to do all the rides and attractions; in fact, you should not expect to do very many at all. You get 2 to 3 hour stand-by lines for popular attractions and that line is most of the day and night -- early morning you can hit some things quickly but otherwise fastpass is key and those run out before noon. Even secondary attractions get over hour lines. You can also get park closings to new quests part of the day. One of the biggest new things you will have to face is the street crowds, particularly at Mk and AK; it just gets difficult to get from one place to another because there are so many people. That walk from Space Mountain to Splash Mountain that you can do in May in 15 minutes or less can take 30 to 45 minutes.
If you want to eat dinner at any sit-down restaurant, you should make ressies well in advance. Even the counter-service places can get 40 minute and longer lines during peak eating times. If you want Fantasmic, do the dinner package or plan to go to the second one at night, the first one fills and you need to get in line early to get in, second one is not a problem.
