Early December is decent for crowds, as pp's have said, filming of the xmas parade can make things weird, Double check your park days and AVOID MK on those days IF you planned on doing Main St/entertainment stuff a lot. The filming takes up the entire street and obviously knocks the daytime parade back to 5pm, and cancels most of the castle shows and Move it shake it that day. Outdoor character meet and greets are also effected since they have to be in the parade filming. We have been in the parks on those days, but stuck to eating at CRT and exploring the other areas and since everyone was over at the filming, it wasn't too bad. Your other option would be to do another park on those days.
Weather is good, I find that if it is a bit chilly, it's a "throw on a sweater and you're fine" chilly. Layers are key here! The mornings you'll need a sweater, but then the sun comes out midday and you're hot until it goes down again, gloves might also be a good idea for the evenings, and avoid sandals and your toes will get cold!