Defictionalization: The whole thing! More specifically, it's a defictionalized Diagon Alley, Knockturne Alley and Hogsmeade, with almost all the shops from those locations (Ollivander's, Borgin and Burke's, Honeydukes, the Leaky Cauldron, etc.) being available to you—the few that aren't, such as "Potage's Cauldron Shop" and "Eeylops Owl Emporium" are presumably removed due to having a limited selection of items to sell—and they even insert little tiny details as well, such as Moaning Myrtle's voice in Hogsmeade's bathrooms and Kreacher peeking out a window in Grimmauld Place. Additionally, the park also put lyrics to Celestina Warbeck's songs, which were only ever alluded to in the books.
Demand Overload: When the ride and the surrounding area first opened, lines just to get into the area were so long that they went outside the park and circled around CityWalk. News reporters had quite the field day.
The Other Darrin: Though they reprised their roles for the Forbidden Journey attraction, neither Daniel Radcliffe and Emma Watson were interested in reprising their roles any further, so in both Escape from Gringotts and Hogwarts Express, they're replaced by two unknown actors.
Sequel Gap: It took six years after the Orlando location opened for Hollywood to receive their own version of the area.
Troubled Production: As mentioned below, J.K. Rowling originally wanted Disney to develop a Harry Potter area, but after numerous disagreements she reluctantly went to Universal to do the job.
Disney originally had taken interest in building a Harry Potter ride. What were their plans? Only one ride for it was going to be made, that would've involved guests using wands and firing at interactive 3D screens (this concept later became the basis for Toy Story Midway Mania!). You can imagine why J.K. Rowling went to Universal instead.
Plans for Harry Potter go all the way back to 1999 when it was originally envisioned as a stunt show attraction.
Early concept art shows a far less ambitious version of the area, that would've not included a Hogsmeade and would've retained much of the Lost Continent's theming, including the Enchanted Oak Tavern restaurant.
The Hogwarts Express was going to have a "leg tickler" effect for the scene when the chocolate frogs get spilled. The effect was ultimately scrapped when the designers felt that it was too noticeable and too loud.
After the midnight showing of Harry Potter and the Deathly Hallows: Part 2 at the Universal Cineplex 20 theater, Universal allowed all of the attendees into the Hogsmeade area until around 5:00 AM, to commemorate the occasion.
Of all things, J.K. Rowling was encouraged to let Universal build Harry Potter by Dr. Seuss' widow, as she had been very impressed with what Universal did with her late husband's work in the Seuss Landing area at Islands of Adventure. In turn, though he never met Rowling, it was the Wizarding World that convinced Satoru Iwata to give Universal the greenlight for Super Nintendo World in the final year of his life.