We just went last year in early December, for the first time in a long time. We found it more crowded than what we remembered from before, but still much less crowded than May or June, and with fast pass it really wasn't noticable except at parade or fireworks time.
As for weather....well it is usually warmer than Michigan. I always say it depends on where you are from...since we are from the same state I will say that I usually wear shorts or jeans and a tshirt with a sweatshirt for nightime. I take gloves but didn't use them last year at all. I have never needed a coat in early december and we have went swimming many times. We have been there several times in early December when I didn't need anything but shorts and tshirst but I know last year I did wear jeans a couple of times and did need my sweat shirt. So it's variable but I think being from MI its safe for you to leave the winter coat and long underwear behind and definitely bring your bathing suit.
After all 60-70 isn't that cold when you are used to 20-30.