For me its the whole atmosphere. The way you feel when you are in the parks. The way the parks are set up - for example on Main ST. You get to see the castle, smells the candy being made and the fresh items from the bakery, listen to the performers, hug the characters, ride the trolley, hear the back ground music etc. its just AWESOME!!!
The way that Disney gets thousands and thousands of people in and out of the parks in such an organized way each day. Whether you stay on or off the premises everything is so organized!
The site of the monorails going in and out of the contemporary and traveling to and from Epcot. It just seems so futuristic! It still takes my breathe away.
The shows, all the details. In fact, I saw Beauty and the Beast on Broadway and the show at MGM is just as good. Wishes, Fantasmic and Illuminations!! They still make my jaw drop!
The way the MK looks at 3:45 after the 3:00 parade--

all cleaned up!!
How clean the parks look at opening!! ( I don't know how they do it when the parks are open until 3:00 AM.
All the details of the rides. They are not just rides, they are experiences for all of your senses.
The variety of rides!! from Test Track, to Mission Space, to EE, RnR , TOT to Soarin" Each offers its own thrill!!!!
The theming of the hotels. We took a surrey bike ride around the boardwalk/ beach club area. It was amazing the way each resort felt different. At the boardwalk everything was so festive, there was alot going on, but just across the lagoon, everything was so relaxing at the beach club.
The fact that I have just a good of time with my kids that I did with my parents when we went with them. Its just ageless fun!!
