We have been in December for the last three years (once during the first week and twice during the second week) and although the crowd levels have increased each year, it is still quite pleasant. Like another posted stated, if you go early in the mornings at park opening, you may be surprised at how empty the park is.
The weather is another story altogether. We have worn shorts one day and gloves the next. Invariably, we have at least one really cold day during our week long stay, but the vast majority of the days are comfortable and around the mid 70s. The evenings tend to be a good bit colder, and if you plan on touring in the evenings, I suggest that you bring some cold weather gear. I always pack a pair of stretchy gloves because as one member of our party stated, those $8 gloves are the very same ones you can get at W***mart for $0.96 and they dont even have a Mickey Mouse on them.
All in all, we love going in December. We love the decorations and can no longer imagine a Christmas season without going to Disney.