As you already know, this is the single busiest week of the entire year at WDW. PP had excellent advice about managing expectations. Also arm yourself with as much info as you can to help make the best of it.
Since capacity closures are common, it's good to know the levels of closure...
The turnstile master computer at each park keeps track of both entries and exits. When a park capacity closes (which is done in stages) there has to be a certain number of people exiting, with the total "in park" below a certain number, before they start allowing people in again.
I am low enough on the totem pole that I have no idea what these numbers are.
There are four stages to park closing:
Stage 1 - The parking lot entrance is closed. You can only access the park if you are using Disney Transportation.
Stage 2 - The ticket windows are all closed (and the automated Ticket Vending Machines are turned off). Unless you already have a ticket you cannot get in.
Stage 3 - Only people with Annual Passes or current Resort IDs will be allowed in.
Stage 4 - Nobody is allowed in.
Note - having restaurant reservations in a park, even with written proof, will not override a stage closing for you.
Also, if you are staying at a Disney owned and operated Resort and have a ticket, you are guaranteed entry into A park. Not necessarily the one you want (unless you want to go into AK). And if you have entered and left a park and do not have a hopper you may not get into anything.
The keys to crazybusy times at WDW...
4. Rope Drop
3. Fast Pass
2. Good Touring Plan
...and the #1 key to having a great time at WDW with thousands of your new best friends??
1. Good attitude and realistic expectations 
Top 3 things to ruin a WDW vacation during crazybusy times...
3. Not making a plan and thinking you'll be able to just "wing it" and do everything you want without ADRs or a clue
2. Not being ok with standing in masses of people or waiting in lines for rides/shopping/food/bathrooms.
1. Expecting to "do it all"
Go expecting it to be crowded and having to wait in line and being prepared and ok with that.
Go with your short list of "must dos" and consider everything else a bonus.
Go with the attitude that you're just going to have a wonderful time just BEING AT WDW!
As for the fun stuff...
Take time to check out the resorts - some of the decorations are amazing and it makes a nice break from the chaos of the parks.
DTD is also a fun place to soak up the holiday spirit. Loads of decorations, music, shopping and entertainment, and there's usually a Santa for photos too.
