I'll be there too! It is my favorite week of the year. It is not the least crowded time of the year, but definitely manageable crowds. I agree that the weather can be unpredictable, and sometimes cold. We've experienced highs during the day anywhere from upper 80s to 40s (or even colder). There have been record cold temperatures when we're there- to the point of needing a sweatshirt during the day, and coats at night (and I'm from near Chicago, so I'm used to the cold). We've also found it comfortable to swim late at night on other days, so just prepare for everything!
Make sure to plan enough time to see all the Christmas decorations and festivities (Candlelight Processional and Holiday storytellers around Epcot, Osborne Lights at DHS, Mickey's Very Merry Christmas Party at MK). Also, the resorts are wonderfully decorated as well- and many of the Deluxe resorts have a Photopass photographer on hand by their main tree (great to know if you're getting Memory Maker, so you can add even more nice pictures to your account). Also, because of the Christmas parties, the MK will close at 7 many nights that week, without Wishes or MSEP.
The first weekend in December they tend to tape the Christmas Day Parade in MK, so adjust your plans around it. It may be interesting for you to see if you never have before; all of Main Street will be taken up by the performers, camera crew and equipment, and any guests who want to be extras, filmed alongside the street watching the parade. However, you will NOT have access to Main Street, all the way through the castle. You will need to hurry through behind the crowd, or through the emporium, and into the rest of the park. You will not be able to get a picture Main Street with the castle that day, and that entire area will be packed and a bit chaotic. However- the rest of the park will still be enjoyable, and the crowds will not be bad at all. There's a good chance the filming will be Fri-Sun of the first weekend (around 5th-7th), but exact dates won't be known until later.