I've been to WDW in December for the last three years, and am going again this year for the 2 weeks before Christmas.
The weather really does vary a lot. In the day, you should expect to wear lightweight jeans or trousers and a tshirt or vest, by early evening you'll want a sweater or cardigan, and if you stay for a fireworks show or similar you will need gloves, a warm coat or big jumper and ideally a hat (it gets REALLY cold).
Of course, some days are hotter than others - last year we went in the first half of December, and there were a coupld of days we wore shorts, but then froze after 5pm!

Some nights are milder too of course.
We have found the only way to do it is to wake up, look at the Weather Channel to see what the low is predicted to be, open the door and feel the current air temp, dress for that, and then pack a backpack with warmer clothes for the predicted low. Then once you're at the park, bung the backpack into a locker and forget about it until you get chilly.
2 years ago it was so cold on some days we left the parks to go back to our room and get warm. Remember that many restaurants will still have the air conditioning on, and aren't as good for warming up when you're cold!
I'd also suggest taking lots of warm sports socks. Take a spare pair each day in case your feet get cold standing around waiting for fireworks, or in case your feet get wet if it rains.
You will also definitely need to bring or buy a poncho - it will rain at some point (although in our experience it clears pretty soon). When it rained on our summer trip, we were happy enough to get a bit wet - that isn't as fun when you get cold later that night!
Have a great time. WDW is at its most magical in December in my opinion! Be sure to watch Epcot's firework show Illuminations early in your trip - there is an extra Christmas finale that is just wonderful!!! And buy a Disney Christmas hat - or you will feel left out!
Miranda