We have been going to WDW for the 2 weeks leading up to Christmas for the last six years and are going again this year!
The start of your vacation should be pretty light for crowds - although we did notice last year that Disney had obviously done a good job with their promotions as the parks were busier than previous years. Do the most popular rides at the start of your vacation. By the 19th you will find there are more people in the parks - the weekend before Xmas Day everyone just pours into the parks and it gets really full by about the 21st (depending on what day of the week Xmas falls on)
It really is worth doing some research about each parks' busiest days and planning to avoid those. Doing the parks in a non-conventional order can make the wait times incredibly small!
Weather-wise, some years have been so hot we have spent days by the pool, other years I have lived in my sweatshirts, gloves, scarves and hats! Bring a range of things that can be worn together or alone.
Also remember there is loads of other cool Xmas stuff besides MVMCP, each resort is dressed up beautifully, each park has a differently themed tree and most have lighting ceremonies come evening time. There is also loads of stuff around EPCOT World Showcase to enjoy that is seasonal - don't miss the candlelight processional. I don't feel it's Christmas until I see that!
Have fun!
Miranda