I've been in the first week of November and a couple times in February - this past February to record cold temps @ The World (think actual FROST). From those experiences, here's what's worked the best for me:
Silk long john tops - basically thin, long sleeved t-shirts. I'd start the day wearing them under my t-shirt, if it got warm enough, I could pull it off and it packed away to nothing.
Pants that rolled up to be capris - I live in these in Ohio. So friggin' handy.
Thin fleece pull-overs or zip ups. Fleece packs tighter, weighs less and does have some natural water repellant abilities. If it does get wet, it will still insulate you and dry quickly.
Thin knit gloves
And - the splurge is a good waterproof shell.... if you don't have one, then a $3 poncho will do the job just fine!
It makes it a little more tricky to pack, since the weather is SO changeable. When we went in Feb this year, on the day we left, the weather was forecasted to have two colder days (highs in the mid 50's) and the rest of the time was supposed to be in the 70's. The next day, while on property, the weather forecast showed no days getting anywhere near 70, and two nights that had heavy frost warnings. So - relying solely on the forecast isn't always the answer.
For the Feb trip, I really felt like I had overpacked - I had my Land's End fleece anorak (that I wear nearly all winter, down to 20 degrees), which is what I wore leaving the weirdly bitter cold of Ohio; I had other thin fleece pull overs and zippies, gloves, silk long john tops, even a couple skull caps. Either myself or other members of my party who were as willing to overpack as I was wore all of it. In fact, those pieces that I doubted the most ended up being the ones that were the most used.
But, no matter what the weather, you'll be at DISNEY!!!!