If you have a waterproof jacket with a zip out liner, that is a great layering tool to bring. You can wear the jacket alone if the temps are cool, and add the liner if it gets cold. Waterproof jackets tend to be good in the wind too - there's nothing worse than a cold wind at Epcot when you're around the lake waiting for Illuminations! I was at WDW during the cold snap last year, and on the coldest days I wore a tee shirt, a zip up hoodie, and a medium weight fleece jacket all layered together. You will want a hat or a hood, and gloves.
The weather can be all over the place. It depends a lot on what is happening up north - if we get a strong cold front through, it will be very cold for a day or two, and then it will start to warm up a bit until the next cold front comes through. That's pretty much the winter pattern here. So you may be bundled up one day and a couple days later, in shorts and a tee shirt! Another thing to consider (hence the waterproof jacket) is that some of these fronts will bring rain and unlike our summer showers that are quick to come and go, it may be drizzly all day - especially if the front stalls out right over us, or passes over and back (which they often do).
Light layers are the best bet, with some jeans and some shorts, a good jacket, and a swim suit if you like to swim (the pools are heated, so if the air temp is OK you are good to go).
