I went the first week in December last year and the weather was gorgeous. It's downright criminal that Orlando enjoys weather like that when up North we are entering a freeze. Regardless, I believe it was in the high 70s and sunny, getting a bit cooler in the evenings. Not too crowded.
The majority of the attractions should be open. It's possible something may close in preparation for the influx of Christmas/NYE traffic, but I would not hesitate to say there will be enough rides open to stay busy and entertained. I would just monitor the ride closures on the calendars so that you are aware in advance.