Until 2015, our family had made 5 or 6 trips to WDW. Each and every one was in June or early July. Last year, we bought annual passes, so we spent both Thanksgiving and Christmas there for the first time. It's busy, more than it is in June. But it's not unmanageable. Only Christmas Day itself in Magic Kingdom is absolutely insane. Beyond that, just use FP+, rope drop, and have a bit of extra patience and you'll have a great time. The crowds, especially in the parks other than MK, aren't horrendous. We had the best holiday season ever and can't wait to go back.
Weather is really variable. We got lucky. Thanksgiving week was in the low 80's. We could have gone to a water park mid-day, but probably not in the morning. Christmas was one of the warmest on record. If I recall, it was 87 on Christmas Day, I was sweating. But I've heard it can get into the 30s at night and 60s during the day during a cold snap in Nov or Dec (Jan and Feb are the coldest of the year).