We spent New Year's Eve 2005 at MK...it was PACKED from about noon on and a CM told my husband they closed the front gates sometime that afternoon. Waits for most rides were terrible...120 minutes for Peter Pan...and fastpasses were gone very early on.
New Year's Day 2005 we spent at AK. It was like having the park to ourselves. Guess too many people continued to celebrate well past midnight. We got on and off rides in over and over, to the point where my husband was saying he didn't want to do that ride again.
Spent the 4th of July at DS this summer...again packed. The major difference between New Year's Eve and 4th of July was that temperatures were bareable in December/January. I'd much rather be packed into a park with 75,000 people in comfortable weather!
As for clothing, I grew up in Syracuse, NY and now live in central PA...don't be fooled into thinking that you'll be fine in a t-shirt in FL in December. On Christmas Day and the day after, I had a t-shirt, sweatshirt, coat, and gloves on and was still freezing. Christmas Day it poured from noon on and the day after there were 15-20 mph winds all day. Both days the high was around 45. VERY CHILLY!!!!
I say go for the trip during Christmas. Although it seemed to get more crowded at the parks as we got closer to New Year's, everyone was in the holiday spirit, the parks were gorgeous, and I couldn't help but have a great time. You will, too!