http://www.touringplans.com/BestDays.htm
On a 1-10 scale, most of October is 4s and 5s with a few 6s. You can compare that with the summer, which is 7 and 8s (9s and 10s around the 4th). We've found the Columbus Day bump to be worse than what Len reports - but he has the data, we are just a sample. (We saw a Splash Mountain two hour wait two years ago that weekend - a six should give you a peak wait time of about an hour).
Some schools have breaks in October - Minnesota is out this year on the 20th and 21st, turning that week into "Twins Fan week" at WDW (the Twins have been doing fairly well the past several years, its the tail end (or right after) baseball season, and there are a lot of Twins hats in the parks that week).
We've been a little shocked by Columbus day crowds - I've been at some slow times, and had gone previously in October when it was much slower. Its become a much more popular time than ten years ago, when October was much more Septemberish.