The national parks are one of the crown jewels of American culture. They manage to hit a sweet spot of ease of access, preservation and diversity in a way that seems to be very rare in the world.
I definitely didn't appreciate this enough until I left the US. I'm Australian, and while we have large national reserves with really cool stuff out there, it doesn't have the same level of infrastructure and access. Now I live in Europe, where there are no where near as many national parks.