I have been that week and we had a great time. We never encountered a park closed to capacity either, I think we got lucky. But then again, I subscribed to Tour Guide Mike and his site had best parks for each day charts, survival tips and lots of other great information. Best $20 I spent that whole trip.
Look at it as an overall experience. Its not just about rides, but the beautiful decorations. Tour the resorts, make dining reservations in advance. You will need sit down meals to give you something that you know you will be able to do each day. Eating in resort restaurants solves the dilemma of "what happens if the park is closed to capacity and I can't get into my ADR". If you do park ADR's its best to do early morning ones, the parks don't typically fill to capacity for a few hours if at all.
Avoid MK on New Years Eve. We went on the 30th instead for the late night EMH. We hung out at our resort most of the day, then went to AK for a couple of hours and actually rode a couple of things, then went to MK. This was an EMH night (and it usually is as they are trying to get people there before NYE instead of on NYE) so the park was open until 4:00 a.m. They did not only Wishes but the New Years Eve fireworks as well. Believe it or not we stayed until 4:00. Between 2:00 and 4:00 a.m. it was one deserted park. We rode tons of stuff and it was just really fun to be there that late. Again, we had slept in, spent a lot of time lounging by our pool so that in essence our day was beginning at about 5:00 p.m.
I think if you go in with an open mind you can still have fun. The fact of the matter is that sometimes you go when you can. If this is your week that you can, by all means go for it. I would definitely stay onsite and I would think about how you want to deal with the crowds. By either going first thing at opening (Mk will open each day at around 7:00 a.m. an stay open until midnight or later) or going to the late night EMH's.
When you can pre book things. Like the Fantasmic Dinner Package, the Candlelight Processional Package and of course ADR's. This week is for planners. If you don't plan you will stand little or no chance of having fun. If you do, you increase your chances. I also think having park hoppers is a must at this time. Because each and every park will be mobbed and you want anything that increases your flexibility.
In the end the extended hours, the entertainment they have going on and the fact that there is something very special about spending Christmas and New Years at WDW can make for a really magical trip. Good luck, I hope it works out for you!