We have been every December for the last 6 years.
We normally go some time around the 9th and fly back out to land in the UK the morning of Christmas Eve.
Early to mid December the crowds are light for Disney. You will still queue, and if you plan badly you may queue for long times, but with good planning you can get on most rides in about 30 mins or under (without fastpasses).
I would recommend looking at the busy day guides that you find on all ears, and having some clear ideas on what your "can't miss" attractions are.
The weather will be way cooler and less humid than summer is in Florida. Temps are normally mid to high 70's, although there have been years that have been warmer, and a couple that have been mid 50's, dipping into the 30's at night - they covered all the plants at Seaworld with blankets that year to protect them from frost!
As another poster said, the pools are heated, and lifeguards on duty. One of the water parks (Typhoon Lagoon/Blizzard Beach) will be open and one closed for refurb. Last year was warm enough that the water parks were quite busy!
We are from the UK, so for us every day seems warm! It's all about what you are used to.
The best holiday attractions in my opinion are:
Osborne Family Lights at Hollywood Studios,
Candlelight Processional at Epcot
Mickey's Very Merry Christmas Party (for the Xmas Wishes fireworks primarily)
Gingerbread House at Grand Floridian
There are loads more, these are just my favourites!
Happy holidays
Miranda