Well, we've been at Christmas twice in the last 3 years, so here's my input.
What to Expect:Expect big crowds, especially at MK. Expect it to get more crowded the closer you get to Christmas. If you go to a park other than MK on Christmas, it will seem empty, but MK is usually packed to capacity.
Advice and suggestions -
*check out each park and the decorations, they each have their own specific style
*go to the deluxe resorts and see the decorations, they have life-size gingerbread houses, carousels made from gingerbread and sugar and things like that which are AMAZING, lots of fun to check out
*consider taking the Yuletide Fantasy Tour to see the various park decorations - this is a really great tour
*If you go to the Osbourne Lights, dont' go for the opening of the day - it will be packed when they turn the lights on for the night, but if you wait a little while to go, the crowds will move through with no problem
*If you want to see the Candlelight Processional, get the package. Around Christmas, it gets crazy popular and the standby line is REALLY long, even hours before the show. But with the package, you can just walk in a sit at the time for the show with little to no wait.
*Check out the Santas in each country at Epcot
*Make sure you pack for any weather - last Christmas, it was below freezing some nights, high in the 30s to 40 one day, and 70 other days. Just be prepared with coats, jackets, and lighter clothes as well...you never know in Florida!
*Have lots of fun and just enjoy the atmosphere!