Our only 2 trips were in September. See my siggie for dates.
We LOVE Sept because the crowd levels are low. However - something major will be closed - you can count on it. The first year we missed Haunted Mansion bc it closes for the holiday overlay and we missed Small World for it's big remodel. Last year -- the toontown coaster (cheezy but my daughter loves it) was closed and we just caught one day of Ghost Galaxy Space Mountain (opening day of that ride - major wait) and Haunted Mansion for one day. Also, the hours are shorter and there are fewer showtimes. That being said, we accept those losses because the crowd levels are so nice. 2009 was definitely more busy that 2008 but that could have been because we went 1 week later and ended our trip at the beginning of the Halloween season.
The weather was very nice in 2008 (a KS girl living in WA now) with temps in the 70s and getting cooler once the sun disappeared. In 2009 we had HOT days with temps in the 80s and 90s.
Feel free to ask any other questions and I will do my best to answer them.