One of the biggest complaints fans have had since the second game is that the Disney worlds have largely been relegated to watered-down retellings of their source material with almost no connection to the overarching plot. III firmly reverses this trend, as all worlds announced so far have stronger ties to the main plot and are not retellings of their respective movies.
Another example is the lack of NPCs wandering around the worlds other than Traverse Town in KH1 and Twilight Town in KH2. This made worlds feel much emptier than they should, with KH2 and 3D in particular being subject to the powerful Narm of crowd scenes with no crowd. Screenshots of gameplay footage from D23 2018 show that this will also be averted, with Sora and gang being shown wandering a populated town area in the Tangled world.
Many people believe, joking or otherwise, that the game has been in Development Hell since 2007. That's not the case; there was never any evidence of it being in development at all until it was announced in 2013.
Also, many people say that the game has been delayed numerous times. This is also not true, as an actual release window for the game wasn't announced until 2017.
Mandy Moore was part of the celebrity cast in the first game as Aerith, but ended up being replaced by Mena Suvari. Fast forward more than a decade later and with the inclusion of Tangled among the Disney worlds, we have a starring character voiced by her. Time will tell if Mandy Moore will return to the franchise to voice Rapunzel.
Ever since Disney purchased Marvel, and when this game was first announced it was exposed that one of the worlds explored would be Big Hero 6 (which is a Marvel property, even if highly modified), people have been clamoring like crazy to have more elements of the Marvel Universe appear (even as "secret bosses"). Now, it's been announced that Square Enix and Marvel have negotiated to make games together.
This meme is now past its prime, given the fact that the trailer announcing a Toy Story world also revealed the 2018 release window.
The game has been anticipated since the release of Kingdom Hearts II in 2005, which has left a lot of people jaded and antsy about the game being released in a timely manner (or at all), with comparisons to Final Fantasy XV (which took a very long time to be released after its announcement, and for a time was explicitly the priority project displacing KH3 development). Even with the announcement of a 2018 release date, people are still joking that the game will be delayed.
Crosses over with Fan Myopia, but the fact that it's also coming out for Xbox One has some Sony fanboys upset that the game series isn't going to be console exclusive (to Playstation) anymore. The fact that many a Sony fanboy practically paraded in the streets over the announcement that the game wasn't going to come out on Xbox One (in Japan, as it was actually clarified), thinking it meant the port to the system was cancelled really cemented that. In actuality, the series has never been console exclusive, as several games have been on Nintendo's handhelds, PSP, and mobile, and Square themselves aren't a console-exclusive company, since they've published games to Xbox, Nintendo, and mobile systems for years now.
Misblamed: The rumor of the game not seeing a release till 2020 stemmed from a misunderstanding of what was actually said. Square Enix gave the release window of this game (and several others) as being within the next three years and news articles jumped on it, claiming the game (along with Final Fantasy VII Remake) had been delayed. Trailers released since emphasized a 2018 release date, killing those rumors.
Older Than They Think: Riku's new Keyblade has been given some flak for resembling a pin tumbler lock key rather than the typical skeleton key template that most other Keyblades follow. "Most", being that the previous Keyblade to use this design template was Fenrir from Kingdom Hearts II.
Popular with Furries: Sora's monster-form in the Monstropolis world instantly attracted some furry fans upon his first reveal, thanks to his cat-like appearance..
Tear Jerker: The Big Hero 6 world will take place after the events of the movie, where the Heartless will possess the Baymax that was left behind inside the portal near the end of that film. What the hell, Square?!
Uncanny Valley: Likely because the game was just starting development, but the first full E3 trailer had some awkward looking facial designs. Later confirmed to be the case as the following trailers show off previous scenes again, with more improved facial structures.
Some people had this reaction to Sora, Donald and Goofy’s designs in the Monster’s Inc. world, especially Goofy's. Sora’s face has been completely redesigned to look like a Monsters Inc. character, Donald is designed with a cyclops eye, and Goofy is designed as a green monster with bugged-out eyes.
A Toy Story world was one thing, since it was the premier Pixar movie. But Monsters Inc? The factory and all, along with both Sulley, Mike and Boo?
Most folks thought that any Organization members they beat in previous games were over and done with. Especially with the reveal that the new Organization is mostly made up of Xehanorts gathered from across time. And then the 2018 trailer revealed both Marluxia and Vanitas are coming back for another round.
The Jump Festa trailer is pretty much winning over a lot of people in the gameplay department. Flowmotion, Shotlocks, and the Classic Combat System all in one package? Hell. Yes.
With Olympus Coliseum being in almost every KH game (Dream Drop Distance is the only exception), many fans were VERY happy to see that the Hercules world of this game will feature all of Mount Olympus and not just the Coliseum.
The Toy Story trailer likewise revealing there will be no trade offs of characters in the worlds. Everyone will be present and accounted for, so both Donald, Goofy and the guest characters will be fighting side by side.
And then came a Monsters, Inc. world. Two Pixar worlds and fandom was through the roof.
The announcement that Utada Hikaru would be returning to the franchise with a new theme song: "Chikai"/"Don't Think Twice".
All of the worlds shown so far have exceptional detail in 3D. This is shown especially well in Hercules' world, which was originally a 2D film.
The in-game graphics of Toy Story's world are almost indistinguishable from the movies. They even nailed the design of Andy's room and the entire neighborhood block! To really hammer the point, even Pixar staff members were shocked at how great the graphics looked! That's the power of Unreal Engine 4 for ya!
The Monsters Inc. world has probably the most ambitious design yet, as not only do they recreate the inside of the factory, but the entire outside of the factory as well!
The Tangled world has realistic water, something that even great 3D animators have trouble animating. And they nailed it.
Also from the Tangled world, Rapunzel's hair. The development team apparently spent three years making sure the hair worked in the game's engine, and it really pays off.