I'm going to have to go with the whole production. All of it.
Nowhere else can you get the diversity of immersive experiences as you can at WDW. The two key words there are variety and immersive. There are four completely different parks with completely different themes, providing many completely different experiences. There are two completely different water parks that each offer unique experiences. There are a couple of hopping downtown-esque areas (Downtown Disney and the Boardwalk). There's a community for living that is unlike most other communities on the planet. There are first rate hotels with themes to rival the parks. And top it all off, there's a transportation system (albeit a bit unpredictable :lookaroun ) to boot!
The whole system is mind boggling! This is world-class entertainment at it's finest. Catering to all kinds of people in today's world... and the idea originally started in the '50s! Who would have thought that such a crazy concept would persevere through the test of time.
And then there's the fact that it all runs so smoothly. Hundreds of thousands of employees go to work everyday to provide US with a good time. In essence, hundreds of thousands of entertainers. That's the largest show I've ever heard of, I don't know about you.
That's what it all boils down to. The Show. The one Design and Accomplishment that amazes me everytime I visit is the fact that I am immersed into such and environment that takes me out of the real world and transplants me somewhere totally different. WDW is like a totally other planet. It feels so disconnected from ours. Only when things like punching Tiggers occur do I actually realize that, hey, WDW IS on this planet. How do they manage to make it feel like that? It's such a mystery.
Because without that wonderful Show with a capital S, none of the other marvels listed would even exist.