We were there October 17th last year through maybe the 25th. The weather was hot, not hot and humid like summer, but we wore shorts and tanktops durning the day the whole time, nighttime was amazing you can get by with capris, a short sleeve shirt because it still is pretty warm. Let me tell you, I dont enjoy Animal Kingdom at all, so we went at like 11am because we have never seen the parade, I never enjoyed this park so much, we walked on everything and i was dying of heat, in shorts and a tank top i was comfortable!
As for the crowds, im not going to lie it was pretty crowded,in the MAgic Kindom durning the day,but nothing like the summer crowds. Also I heard alot of people saying that the European countries were on their "break", so it was probably more crowded because of that. We used fastpass a couple of times and waited on anything that said 20minutes or under cause it usually went quicker than that. Animal Kingdom was walk on everything, MGM we waited maybe 15 minutes for Rockin Rollarcoaster, Epcot wasnt bad either considering it was the wine and food festival, we walked on all of those rides too. Magic Kindgom durning the day was a little rough.
The best tip i can give to you, Is go to Mickey's Not So Scary Halloween Party, we walked on all the rides no waits whatsoever, we had Magic Kingdom done in a night, all the rides were open pretty much,i dont think there was one ride that was closed that i wanted to go on, it was wonderful! I'd recommend doing it twice if you dont mind spending the money, it was well worth it!