Re: How's the mouse in Dec?
Well, your first few days are going to be crowded since you are arriving on Thanksgiving weekend but after that Sunday the crowds should start thinning out. December 1-15 is one of Disney's slowest times.
Park hours shouldn't be an issue. There are always Magic Hours going on (I love when AK stays open until 8pm). I have also never run across a time when animals are not out.
Re: How's the mouse in Dec?
Yep. Thanksgiving and the weekend immediately after will be wicked busy. You'll have to plan what you want to do and make good use of EMH and fast passes. After that things will ease off, but it's not going to be exactly quiet. MANY people know that the first 2-3 weeks in December is a "bit slow" (or so they thought) so they head to WDW, DVC members in particular.
Have a rough plan for what you want to do, but remain flexible. If you want to have a table service or buffet meal in any WDW resort or park restaurant, make an ADR. Other than that, you should be good to go!
Re: How's the mouse in Dec?
Our first trip to The land of Magic in 2003 was in December and we found crowds to be ok, the weather was warm during the day but a little chilly in the moring and the evenings.
We didn't have any trouble with the hours at the parks, I think as you are staying onsite your be fine.
Hope you have a amazing time.