We have been to WDW the last several years in December. As you have read above, it varies. Be prepared for anything! One year (2006) the day we arrived it was 84 and gorgeous and sunny! We were swimming, etc. Each day, gradually, the temps went down until the last day of our visit, we were in sweatshirts with hoodies over them, with the wind blowing and we were very cold. In December, you just never know! I know it's nice to be able to pack lighter (i.e., shorts, t-shirts) for "Florida", but it's worth having the other stuff with you, too, because you don't want to have to go out and buy heavy stuff. It gets costly.
For us it has been a little easier to do that since we come from the Philadelphia area and it's cold up here in December, so we have to be dressed in it when we're leaving anyway. That allows us to "carry" some of our heavier stuff. We layer our clothes so we can take it off when we get to Orlando.
I wish you only great weather when you go!! (Us too ... we're going for Christmas again!!)