I was at WDW last December (first week) and the weather was great. Over an eight day period, rain only interupted our plans once. I think the rainy season ends in Nov. It was warm during the day (enough for shorts and swimming) but it did get cool in the evening (usually wore light jackets to be comfortable). So you may end up packing more clothing than you might anticipate.
Crowds were fairly light when compared to summer. Usually didn't have to wait more than 20 minutes to get on a ride. Saturdays in MK was the most crowded as many of the locals come to the park. Also (don't know if this is still true), the evening parade in MK was held only on Saturdays during the off season, so more crowds were definitely bigger, with longer waits.