Let me try to give some info:
I don't know when in December you plan on visiting, but usually the first two weeks of December are not too full. As the days get closer to Christmas and New Years, the parks tend to get very packed.
Weather-wise, December can be very cool in the 60's and sometimes in the 50's. We're known to have showers here once in awhile, but it can be fun because it scares the crowds away :animwink: .
Christmas decorations and lights will be up, so take the time to look around, especially Mainstreet and New Orleans Square. Another New Orleans recommendation, the bead necklace toss (ala Mardi Gras, but tame). It's fun and people always get involve. Warning: To get a necklace you may have to do a funny dance) :drevil:
Not to miss Christmas attractions: The famous Nightmmare Before Christmas Holiday in the Haunted Mansion and IASW Holiday. Both big crowd pleasers. If you can, try to pick up a fastpass for both attractions. Believe Holiday fireworks is not to be missed and after the fireworks, it snows in Mainstreet.
It's all I can think of now. I'm sure others will come in with more ideas.