Is it the theming? Quality of attractions? Something entirely different? I want to know.
The answer is all of the above, although some elements you mentioned are more important than others.
The most important aspect of a theme park--indeed the very thing that separates a
theme park from an
amusement park, is "theme." This does not mean that the entire park needs an over-arching, coherent theme in order to be considered great. The lands of
Disneyland, for example, do not follow a single theme or cohesive story, yet the park is perhaps the single greatest triumph of themed design ever built.
Islands of Adventure is another great example of a good theme park that does not necessarily have a constant theme throughout.
So what makes these
Disneyland-style parks so great? In my mind, it's believability. When walking through Frontierland, guests feel as though they are "in" Frontierland. More than just some area in a theme park, Frontierland is a living, breathing place full of its own style and appeal. It feels real.
Incidentally, while I have not had the opportunity to visit
Shanghai Disneyland (yet), based on the images and videos I've seen, I think that the feeling of believability is what is missing from that park. In part, this may have to do with the park's size--SDL is so massive that the gaps which exist between areas is kind of jarring (again, solely based on images/video I've seen).
The bigger issue with
Shanghai Disneyland however, is that next to none of the park is based in reality. The Tony Baxter-led 1983 redo of Fantasyland was steeped in Bavarian-style architecture rather than in the imagery of movies/storybooks. It was influenced by real-world design; by the places and peoples who created the stories that Disney's animated classics were based on, rather than by the stories themselves.
In contrast,
Shanghai Disneyland's Fantasyland feels artificial. The architecture and design of the land appears to be based on assumptions of what storybook/European fairytale villages would look like, rather than on an actual European "fairytale" village. This issue might be best summarized by this sentence from the
Wikipedia article on Shanghai's Enchanted Storybook Castle:
"The look of Enchanted Storybook Castle was inspired by many other Disney Castles, including Cinderella Castle and Sleeping Beauty Castle."
So Enchanted Storybook Castle was inspired by "Disney Castles," meaning fictionalized structures custom-built for a theme park. Meanwhile, the castles that inspired Enchanted Storybook Castle--Cinderella Castle and Sleeping Beauty Castle--were inspired by actual, real-world castles and palaces from old-Europe.
Imagineering Legend Herb Ryman, for example,
based the design of Cinderella Castle on:
The
Château d'Ussé,
Fontainebleau,
Versailles and the châteaux of
Chenonceau,
Pierrefonds,
Chambord,
Chaumont,
Alcázar of Segovia,
Neuschwanstein Castle in
Bavaria and
Craigievar Castle in
Scotland. Other sources of inspiration include the
spire of Notre-Dame de Paris, the
Moszna Castle in
Poland, built in the 18th century, and the
Tyn Church in Prague, Czech Republic, built in the 14th century.
Sleeping Beauty Castle, meanwhile, was inspired at least in part by
the Neuschwanstein Castle in Bavaria and Chenonceau Castle in France. Incidentally, Sleeping Beauty Castle's basis in Bavaria's Neuschwanstein Castle is likely what led Tony Baxter and his team to select a Bavarian-design for their 1983 New Fantasyland project--again, because doing so would help both the castle and the Fantasyland "village" appear real. What appears more natural to the human eye than a Bavarian-inspired castle surrounded by a Bavarian-inspired village, after all?
Wow, this post got very long very quickly. I had intended to discuss some of the other elements that I felt made a theme park great, but that might have to wait until another day. Ultimately though, I believe theme--specifically, the believability that theme brings to an area/land--is the most important aspect of a theme park. Once a park's "theme" is set, the other elements (which are still important!) begin to fall into place.