Prior to this trip, I have only gone in the summer when it's really crowded. We were in the parks from 10/26-10/31 and it wasn't as crowded as in the summer months. I think of it as being along the lines of moderate crowds and workable. The lines were really no more than about 20 minutes, with the exception of maybe about 2 rides.
Madelinesmum is right about parade time. The parade route is always conjested prior to the parade coming through and after. It was very difficult for us to move around, especially with a double stroller.
MNSSHP on halloween night was terrific! Our first time and unfortunately, our DS4 fell asleep after being there for about an hour, hour & a 1/2. The parade is done twice and it was highly recommended by a CM to catch the 10:30 pm show where it's not as crowded. The fireworks show, HalloWishes, was shown on 10/29 (which was an EMH night for us) and again for MNSSHP. Most of the major rides are open during MNSSHP. If you plan on doing MNSSHP on halloween, make sure to get your tickets early. When I ordered them on the phone, I was told it's usually the first night to sell out.
October is also when The Food & Wine Festival is happening at Epcot. Make sure to hit Epcot during the week while the F&WF is going on. The locals usually come on the weekends and it is very, very crowded. One DISer posted that it felt like being packed in like sardines.
The weather was great. It was in the high 70's to mid 80's during the day and in the mid to high 60's at night. Make sure to bring a sweatshirt and pants to change into if need be for the early am and evening. It only rained one morning but cleared up prior to our leaving for breakfast at around 7 am.
Hope that helps you out with your planning.