I went last year on Halloween night and it too was sold out. It always sells out.
It was our first
MNSSHP but we also did
MVMCP 2 times as a comparision on crowds etc...MNSSHP was MUCH more crowded.
But if you go knowing this and just enjoy the atmosphere, trick or treating, get your photo taken, and enjoy the parade (I'd wait for the second one) then you will have a blast.
I felt the rides were crazy but I'm talking about Fantasyland...I don't know how other areas were. We rode HM right away and didn't have a wait. We got to ride Goofy's Barnstormer twice in a row with no wait either so I think it depends on where you hang out.
We also got the photo over with right away too so we wouldn't have to worry about crowds or getting dirty or cranky etc...
The trick or treat lines appear long but they go SO fast that it was no problem at all. They have lots of good candy and each places give something different!
Unlike MVMCP both parades were crowded. If you go to the second parade, I'd get to the parade route at least 1/2 an hour before. If you go to the first, then you need to line up ASAP!
I believe many people were just walking around enjoying the atmosphere since it was Halloween.
I think you'll have a blast no matter what!