Hi! I'm joining in.
I've gone to WDW twice during the holidays (arrive Dec. 26 leave Jan 4 or so).
A few observations and tips for that time of year:
1) Definitely have park hoppers; if a park (MK or Epcot) is too crowded for your liking head over to AK or DHS for a break. You can always go back in the evening and usually the crowds will thin out by that time.
2) Plan to arrive at the parks early bc there will be an influx of locals and off-site guests. Disney has different capacity level closings and the best way I have found to keep informed of what is going on is by taking the Disney buses. One year, we drove in to MK, but it was closed - the gate attendant said that hotel guests could still get into the park but only via Disney transportation.
3) We are not much for drunk and disorderly persons, especially on NYE. We found that Epcot on NYE day is actually quite doable, later in the afternoon is when the crowds build up. Then we head over to DHS for their party. It is fun (dance parties, etc.), still serves alcohol if you would like to toast in the new year, and the crowd isn't as large.
4) If you go to MK on Dec. 30th, the fireworks display is the same as NYE since they are doing a test rehearsal; it may/may not have the perimeter fireworks, but they do the countdown, etc. The crowd is also more manageable this night too.
5) I have heard it is best to avoid MK on Christmas Day itself. I have not been there for that day, but I would think it would be incredibly crowded.
6) Develop a game plan and use fast passes as much as possible. Know that you are not going to see everything, and it is a perfect reason for another trip! Have a list of must-do's, and a few backups (eg. oh, HoP is starting, let's pop in there to get out of the parade crowds).
7) While you do not need the dining plan, you will need ADRs. Tusker House has a fabulous breakfast at AK.
8) If there is time, check out some of the resorts' decorations, especially GF's giant Gingerbread House.
9) The Candlelight Processional is amazing!!!! It is also extremely popular. If you are thinking about seeing this, book a dinner package otherwise you will be waiting in the standby line for hours, to *Possibly* get in the theater.
I hope this helps!