Crowds, particularly at the Disney parks, will be at park-closure levels. We were in Orlando from December 23 to 30 this past Christmas, and Magic Kingdom was experiencing closures from Christmas Eve day on. Even Downtown Disney felt mobbed......tons and tons of people milling about everywhere.
Expect tons of traffic, tons of people, tons of lines. And not only at the Disney parks, either. We couldn't get a table at Olive Garden on I-Drive on at least two nights of our vacation because there were so many people waiting to be seated.
A 15 minute drive to the Poly took us an hour, as cars were stop and go as far as the eye could see.
That said, if you PLAN, PLAN, PLAN......it can be amazing. We avoided all Disney parks, added in a few less popular (but equally awesome) attractions, and had an incredible time. You can check out my trip report (the link is in my signature) for more details on how we spent our week. Christmas Day itself was spent at SeaWorld, and it was fantastic.....no big crowds, no huge lines, nothing like we experienced a few days later. For any park days (whether SW or Disney or Universal) I would recommend staying onsite at the park of your choice (Universal and SeaWorld properties offer their onsite guests free front-of-the-line passes which are a Godsend), and planning on being to the parks at opening to enjoy what you can before the masses hit.
We had great weather overall. A couple of cooler days mixed in, but warm enough to hit Aquatica on Christmas Eve Day
and even "hot" on at least a day or two.
If you have any specific questions, don't hesitate to ask!