We've been at WDW for Christmas-New Year's Day three times now.
The parks will be the busiest they get all year. Period.
We arrived one year on the 20th and were amazed at how the crowds increased and the general mood changed after the 23rd or so. Instead of being festive and fun, people got pushy, grouchy, and generally way too serious about "getting our money's worth". I saw a major meltdown by a teenaged girl because her Coke was gone! Maybe it's understandable to a point, I know lots of families sacrifice a lot to go and may even have just driven 20+ hours nonstop to get there, but still.....
You simply won't get to do all you would like to and moving around the parks will take all your savvy and park tricks just to do so. We know this, so we adjust our expectations accordingly.
Well, almost, because the parks often stay open later, sometimes way later to give guests a chance to enjoy a few more attractions. So if you can stay until 1 or 2 AM you'll have fewer people to deal with.
Depending on the weather, Splash can be a walk-on. Most people won't do a water ride at cooler December temps even though much of the water works are not turned on when it's "cold". We have also had very short waits for Big Thunder, again if it was colder out. It pays to be a Canadian at those times.
Consider trying Mickey's Very Merry Christmas Party. Compared to the Halloween Party this one gets mixed reviews. If you do go, focus on the shows and parade more than the attractions. I still like to attend if we're down while they're holding them.
Above all you're at Disney at Christmas and it is magical (even though everyone there may not be)!
I'm looking forward to my first ever (hopefully) Green Christmas!!!! 
