Have loads of fun here. Typically the first week of December is quiet time I think, if you ask this question on the Theme park boards you might get some answers from people who have been during this time
i went there last thanksgiving,pretty close to the time of year you are going there, since i was there in the holiday week, i expect my week to be busier that yours, and it wasn't that bad when i went.
As for the temperature, the days where warm but the nights cooled down as soon as the sun went down(so do the rides that you will get wet on in the 1st part of the day, so you can dry off before the sun sets!) or bring a change of clothes. Here is a link that will show you the average temperature throughout the year there: http://www.cityrating.com/citytemperature.asp?City=Orlando