We were there this past Christmas for a week and we had a great time. The MK was packed but we didn't find the other parks too bad. Like you said, get there early, have a plan, use fast passes wisely, and be sure to make a PS for any meal you really want. Oh, and pack a little of everything. Some of our pictures have us wearing shorts and in others we are wearing winter coats, hats, and gloves!
I agree with toystorymom. We've done Disney between Christmas and New Years for the last two years. Last year we stayed two weeks and did find it nicer before Christmas Day than after.
Don't save the Magic Kingdom until your last day as that is the park that reaches capacity and closes the most often. I did notice this time that it would close for an hour or so, then reopen, then close again. I always thought once it was closed for the day, it was pretty well gone for the day.
We just bypassed lines that were two long and enjoyed ourselves.