Here is my tips for Christmas as WDW.
Illuminations: about 1hr 15mins before the start pay attention to which way the wind is blowing. Then go the opposite side of the lagoon. The holiday tag throws off about 2800 shells in only a couple of minutes so it can get pretty smokey.
Wishes/Holiday Wishes: Main street is packed, stand back towards the train station and as soon as it's over, head up the train station and wait it out or hop on the train if it's not too long after so you don't get "crushed" in the crowds.
MVMCP: If attending always try to catch the later shows, the early ones are most crowded and a lot of families leave right after Holiday Wishes.
Characters in Holiday Costumes: They are all over the place but most noticable in Animal Kingdom. Makes for a great christmas photo op.
Picture taking of lights: All the christmas lights can cause some pictures to come out blurry if you take the picture too quickly. If using a digital camera, check your photo if you can really quick, otherwise you may miss that Magical Moment.
Osborne Lights: You really have to be there for the light switch ceremony. Yes it's crowded but WOW what a way to start the evening. All those lights, snow and musical.
Holiday Merchandise: I have found over the last 4 trips there is certain stores that have items that others do not and you really have to pay attention. Some of the Animal Kingdom shops have some hidden treasures.
The weekends are the busiest and it slows down normally during the week days, but with disney throwing all these promotions out there, the parks are becoming more crowded and there is less of a "Slow Season"
Above all enjoy your magical holiday. (afterall it's like having christmas twice in the same month)