I was at Disney for Christmas last year and in 2005. The weather is "sweater weather" during the day and you'll need a jacket in the evening. Last year, I wore velour hoodies a lot.
I'm planning an early December wedding to try to avoid some of the crowds and make sure that all of our guests get back home in time to enjoy a "normal" Christmas.
As far as anything special for Christmas goes, I'm thinking that we'll do a nice dinner at California Grill the night before the wedding (which will be the "escape" type) and, of course, time it so that we can watch Wishes from the restaurant. Then we're aiming to do the actual wedding on a Tuesday late-morning or early-afternoon. Instead of a traditional reception, I want to get tickets for all of our guests to join us for Mickey's Very Merry Christmas Party later that evening. Kind of like "The wedding's over . . . now it's Christmastime!"
For those of you looking for something special for Christmas Eve or Christmas Day, I would recommend doing the Candlelight Processional in Epcot. They have a celebrity narrator (last year was Gary Sinise) who reads the story of the nativity and a huge mass choir. It's a really nice way to celebrate the true meaning and spirit of the holidays. They usually have a dinner package that you can do that will get you priority seating at the front of the crowd. Another fun thing we did last year was a Christmas Eve dinner at Ohana . . . yum!