We really enjoyed our early December trip. I would never say to avoid ALL of December, just the part once kids are getting out of school and people are starting their vacations. But the time after Thanksgiving and before, say, the second half of December, can be quite nice. We got a good rate at a hotel, the airfares weren't awful yet, and the crowds were light enough.
As for the cast parties, they do close DL early for those (it's on 2 nights, I believe) but DCA is open a bit longer. And with the Candlelight Processional, Main Street is crowded for it, but if you're already well inside the park you'd probably be fine, or you could plan on being in DCA.
You'll see the CP on the calendar once the December calendar is up, and the CM parties will be seen in early closing times in early December.