If you're going prior to Christmas Day, crowds will be lower, depending when in December your trip is. The first 2 weeks of December are relatively light, crowd-wise. Starting about the 15th, there will be a noticeable increase in the crowds each day, culminating with max crowds on Christmas Day. The week between Christmas and New Years is THE busiest week of the year. If your goal is to ride a bunch of rides, this is not the time to go. If your goal is to enjoy the decorations, entertainment, and ambience, it can be a very magical time. It's all about setting expectations. Take advantage of early entry. I'd recommend considering a Party. Expensive, but also a unique experience for the holidays with lower crowds and more ride time.
If the crowds start to get to you (and even if they don't), take a break one afternoon, and check out the Deluxe Resorts. Each of them has their own themed decorations that are worth seeing.
As mentioned, the weather can be anything from August type heat and humidity to cold and rainy. Plus, it can do all that in the span of a week.
Steve