MNSSHP at WDW includes a special Mickey's Boo To You Parade which runs only at the party.
Before the parade a headless horseman gallops the route. It is spectacular. The parade itself has all kinds of different floats and it had the grave digger squad performing routines with their shovels. They were really good too.
There is also a special fireworks display called Happy Hallowishes that is better than wishes itself. This is also shown only at the party and the lights that shine on the castle change to a creepy color scheme.
There are also two dance parties One is in fantasyland, the other in Tomorrowland. The Fantasyland one had characters coming out and dancing every 3-5 minutes on a rotating basis.
They also have character trails where the characters come out as well as trick or treat candy stops. Some of the character stops have characters you don't see much like the queen of hearts. Others had the characters that are always out but they were in Halloween costume. Eeyore was dressed as a mummy for example.
They also had the villain's show at the front of the castle. Afterwards they would mix and mingle a bit. We were disappointed in this because they were simply mobbed and there wasn't much time for pictures.
They played special music and they had some nice decorations and effects.
The rides were all nearly empty.
The party starts at 7, but most of the time, they let you in to the MK on your party ticket at 4 pm. We entered about 4:30, rode an empty Buzz 3 or 4 times. Then we ate at the counter nearby for Dinner, Cosmic Rays and were in Tunetown to see Mickey and Minnie from the off. Mickey actually came out to his garden early so we got to see him before the party was supposed to start.
About 80% of the kids were in costume and about 50% of the adults. Those in costume usually looked more into it and looked like they were having more fun.
We found the parade and fireworks alone well worth the price of admission. But if you're not into parades and fireworks, I don't believe the rest of it would be worth the price of admission on its own.