Lots of things. When you are in Busch Gardens you never for a second think you are in any other place other than a theme park. Ditto for Six Flags parks. When at Disney you are cut off from reality and society and life is different there. Going to the Magic Kingdom truly is an experience. You park your car, you take a tram, you take a ferry/monorail. You are shut off from society and it feels great. There is happiness everywhere you look, you are taken back to your childhood at every turn and you are pampered to death. Not to mention the million photo shots you can take.
Throw in stuff like Spectro Magic and Wishes at the end of the night. I mean honestly how can that fireworks show NOT give you goosebumps? Ever taken the ferry back at night? It is something you truly need to experience, look back at the castle, it almost feels as if you are leaving your home, your friends and a dream.
I am a theme park junkie and out of every park I go to only Universal can come close to the feeling and nostalgia Disney can give you. Even then Disney is still the clear victor in that regards. Plus it's not just that but you wouldn't think Haunted Mansion would attract your attention but it does. Every time I'm on it there is something new I missed. Where else can you go and feel like you are literally in a dungeon boarding a pirate ship? Again, the only place that comes within a country mile of Disney in that aspect is Universal and it's still behind.
So that's my answer