Weather: Some days it will get into the 70s during the day, other days it won't get out of the 50s. You really need to bring a little bit of everything and watch the forecast carefully. Remember to wear layers. If it's gonna hit 70, we normally start out with a sweatshirt and t-shirt in the morning, and then strip off the sweatshirt as it warms up.
At night, it will DEFINITELY be cold. After sundown, plan on jeans, sweater / sweatshirt. If you can, bring both a light jacket and your winter coat with gloves and a hat. No, it doesn't get that cold every night, but there WILL be nights you'll be happy you had the winter gear with you. And, if you don't bring from home, you'll end up paying Disney prices for brand new hats and gloves just to stay warm.
As stated, all of the pools are heated. Our threshold is normally 70 degrees and sunny. Anything cooler than that and the air temp will really hit you once you get out of the pool. The best time to swim is when the sun is high. By the time it starts to set behind the trees (which could be as early as 4pm), you get that cool breeze and rapidly falling temps.
Don't forget to prepare yourselves for the ride rehabs in January. I believe Dumbo, Astro Orbiter, and The Land pavilion at Epcot will all be closed early in '05. It's a Small World has been closed for rehab for quite some time and does not re-open until March.
Other than that, it's a great time to visit. Very low crowds.