We visited last year in early December, and I can tell you it was wonderful weather. Only one day of rain! High temps in the 70s! It was perfect.
As mentioned above, a light jacket or sweatshirt for early morning and late evening is a must. On these boards I had read suggestions to get take hats and gloves. It was by far one of the best suggestions we picked up here. We only wore them one night, but that night was perhaps in the high 40s, and MK was open til midnight. We practically had the park to ourselves as everyone else was shivering, and heading back to the hotel. We were actually riding Splash Mountain, Big Thunder Mountain, Hanuted Mansion and all of the other rides with no waits, all while comfy in our Mickey hats and gloves - only $5 a set from Wal-Mart.
I remember that night so vividly, we (unexpectedly) had the opportunity to watch the late night Spectromagic parade with absolutely no crowds to fight, the parade characters actually interacted with us - it was an absolutely magical night!
We have a baby due in Nov, otherwise I would consider returning again at that time later this year! I guess there is always next year!