First week of October coincides with the opening of Haunted Mansion Holiday, and that affects the crowds. Last year they also did a resort-wide celebration called "Disney's HalloweenTime" with lots of special decorations and show, and that *really* increased the crowds in October. Haven't heard if it is returning this year, but it was very popular so I'd be surprised if it doesn't. Still, the worst crowds will be on the weekends - Friday-Sunday - and weekdays are still not that busy.
The weather in October is generally pretty nice - if we get a Santa Ana condition it can be VERY warm (high 90s and even into the 100 range!), but it's dry. Normally it's probably low 80s during the day and cools off rapidly (into 50s and 60s) at night - typically you'll want a light jacket for the evenings.