My now fiancee and I went the last week of this past October and it is probably our new favorite time of year to go. The weather was basically perfect for the 11 days we were there (not so much up here in the North East since a freak snowstorm forced us to stay an extra 3 days... drats). I think it rained one morning and the temps were basically in the mid-80's every day. I think there were 2 days that we spent just at our resort (AKL) just hanging out by the pool or in the hot tubs. Over 2 weeks durring that time of year you could deffinately get some good use out of the Water Parks and More option if you wanted.
The crowds were not bad, as the PP mentioned, but it seems the gloriousness of this time of year is starting to catch on. I used to hear that the crowds were non-existant this time of year, but that doesn't seem to be the case any more. However, the crowds for us were still very managable. Your biggest issue will be having to deal with Columbus Day weekend, which will spike the crowds for a few days. Also, the WS will be a nightmare on the weekends due to F&W. However, the corwds for F&W are pretty low durring the week, unless there is a big concert. Durring the week, the night we saw Boyz II Men was pretty packed, but the night we saw the Gin Blossoms it was pretty low (Gin Blossoms were by far the better performance, BIIM spent too much time plugging their new album). But then again the concerts are free so I shouldn't complain.
MK really gets into the Halloween feel. MSUSA looks great. It is a much different feel than mid-January or May or August or Christmas/NYE. There seemed to be extra characters and PhotoPass photogs around (probably due to MNSSHP). MNSSHP was something great to do, but we are in no rush to do it again. There are some things about it that are great and other things that I think they need to re-think. I'm still walking around singing the Boo to You parade song.
To sum it all up, the F&W is enough to sell us on this being the best time of year, but there is a lot more going on. Not to mention that the weather was great and the crowds were managable.