The first 10 days of December are my favorite time to go, I think I've done 5 trips in that time frame, and have one planned for this December as well. The weather is usually perfect for this New Englander, though we have run into cold snaps a couple of times; I just make sure I check the weather the week before so that I know how to pack.
Crowds are usually quite manageable, though I'm starting to worry about this year as there seems to be so much pent up demand for travel. I booked a room only so far (waiting to see if APs come back, and Southwest hasn't opened their schedule for December yet), and 2 weeks later the prices more than doubled for what was available. At the moment I'm grateful I booked when I did. I actually have 2 reservations currently, one at CSR, and one at the Swan, not sure which I'm keeping yet. I really want to stay at Animal Kingdom Lodge, but the hotel isn't open yet. I'm keeping my fingers crossed that Disney opens more resorts before the end of the year.
Everything is decorated so beautifully, Christmas is my favorite. The Christmas party is worth doing as well IMHO, my last trip was in December of 2019, and even with a sold out party the crowds weren't too bad. If they don't end up doing a full party this year I may skip, not sure yet.