Attention to detail, amazing theming, friendliness, safeness, cleanliness, happiness, organization, fantasy, all-age design and accomodation, freedom from -isms, and a place that allows folks to drop the walls, shields, roles, masks, and attitudes they use in the outside world.
No place for me can mix elements of all of those things the way Disney can...mostly because no other place was founded by and inspired by the peerless vision of one particular man whose name the parks bear.
It says something that I am a person who has no interest in theme parks...I don't go to Universal or Busch Gardens despite both being within the same general distance to me...yet I go to Disney World 5 or 6 times a year.
I don't care for action rides, I don't care for carnival atmospheres, I don't find crowded theme parks to feel friendly, worldly, and accepting, I don't notice much in the way of intricate details and design at theme parks, and I don't find the employees quite as enthuisastic, happy, friendly, understanding, or well trained for the most part. Except at Disney.
That doesn't mean that there are no faults at Disney. They can get the theming wrong every once in a while. Some visitors to the parks might occasionally break the mold and be rude, judgemental, or hateful. Some cast members might be grumpy or disinterested. But these things are much more rare at Disney than any other place I've ever been. And even when it misses on one count, it hits on all other counts, and that more than makes up for the occasional shortcoming.