The days of an off or slow season are a thing of the past. Don't expect to go in to any park and just be able to walk on any ride you want.
The main headliners (E-Ticket rides), i.e. Test Track and Soarin' in EC, Splash, Space and Big Thunder Mountains in MK, Rn'R, TSMM, ToT in DHS, EE and KS at AK will still carry a 30-90 min wait, while other rides will see lines of anywhere from 5-20 minutes. We went this past year durring the last week+ of October. The days MK has a MNSSHP were actually the best days to go, crowds were non-existant until about 4 or so when they start letting partygoers in. EC has larger crowds Fridays-Sundays due to F&W. Also, the days were a more popular musical performance plays will be more crowded than other days. For example Tuesday when Boyz II Men performed was much more crowded than Thursday when the Gin Blossoms played (and the Gin Blossoms were 1,000,000 times better, IMO).