Our first trip in December was the second week of December 2001. It was like a ghost town (I'm assuming because it was only a few months after 9/11). EVERY ride was a walk on and there was hardly anyone walking around. It felt very strange. We've gone several times in December (varying from the week after Thanksgiving to the second week of December) since and it's definitely more crowded every time we go. However, it's not Summer or Christmas week crowds. Rides are generally 15 min.-40 min. (of course, rides like Toy Story Mania and Soarin' can be 1-2 hours), with some occasional walk-ons.
Weather is a different story. We've had beautiful weeks of vacations in December where it's 70-80 degrees and sunny the entire time. Then we've had weeks that were 45-55 degrees and cloudy/rainy/drizzly pretty much the whole time. It really is just a chance you take to see the Christmas decorations and festivities, unfortunately. We're going December 3-14th this year and I'm praying not to have the horrible cold weather they had last December! I'm trying to leave that behind!

Packing can be difficult during this time as it seems like whenever I just pack 1-2 pairs of jeans and a sweater or 2, it's cold/chilly every day and I want to wear jeans and sweaters every day.
We're going to 2
MVMCP's this time. While we do enjoy the Christmas parades and events, we generally use this time to get to be in MK at night and most of the rides are walk-ons. That's one of my biggest complaints about going in early December is that during the week, even with EMH, hardly any parks are open past 9 p.m. (which I guess is fine if you have little kids or don't have a desire to be at the parks anyways until 11 p.m. or later).
One of our favorite Christmas activities is to go to Epcot and get some drinks or snacks and wander around World Showcase listening to the Santas Around the World. They generally "perform" every 15 minutes or so (the times guide when you enter the park will have that info) and it's neat to learn about Christmas in different cultures/countries.
We used to the Candlelight Processional and I believe everyone should try it at least once, but we're not huge fans. We typically avoid it on subsequent trips (unless someone we REALLY like is presenting the story). If you do go, be sure to bring something to sit on (even if it's just a sweater or jacket). The seats are metal (which means cold in the evening!) and uncomfortable.
No matter what, I'm sure you'll have a great time! I read people's trip reports from last early December and even though they have scarves wrapped around their heads, thick winter coats, and gloves, they still had smiles on their faces

.