On one hand the revelation that the Osaka team behind Birth by Sleep and Dream Drop Distance is developing the game rather than the team behind Kingdom Hearts and Kingdom Hearts II has been a sore subject for many. On the other hand some are actually relieved to know that the team behind Birth by Sleep is taking care of it, or at the very least, eased by the fact that they don't have to wait for Final Fantasy XV to be released for production to begin (as was initially thought).
In a 2014 interview with Nomura, when asked about Frozen being in KHIIInote stemming from a misinterpretation from an interview with Tai Yasue, Nomura responded that "we haven't touched upon it at all, and there's been no talk about what we'd do if we put it in for now." While a lot of fans requested it, many others are affected by Hype Backlash, and are glad the world is not appearing.
Even amongst Frozen fans there are fears on how the series will adapt the movie, such as if Elsa will have villainous qualities or who will voice them. Rumors of musical numbers have freaked out fans even more, as they bring to mind the dreaded Kingdom Hearts II incarnation of Atlantica.
This ties in with the inclusion of the Big Hero 6 world. While most people liked that decision, many Frozen fans complained about that film's inclusion in the game before Frozen, while others have commented that had Frozen's had enough attention for now and it's good to see that Disney hasn't forgotten its other new films.
The confirmation that the Command Deck from Birth by Sleep, Re:coded, and Dream Drop Distance has been removed has some glad, thinking of it as overly complicated and considering this a return to basics, while others see it as a significant step backwards after the developments of the handheld games.
Should Master Xehanort indeed be the True Final Boss? Many fans believe it would be appropriate closure and are tired of all the Man Behind the Man moments, while many others believe it would just be a boring Untwist and are tired of the way he's been implemented as the Greater-Scope Villain (especially as of Dream Drop Distance).
Hilarious in Hindsight: Mandy Moore was part of the celebrity cast in the first game. Fast forward to 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.
To start off with the most obvious example, the very fact that this game has taken forever to be made and released, especially since the last numbered installment was released in 2005 in Japan.
Footage shown off of an in-development build by Disney in October 2013 showed the game's graphics in a state that some say doesn't look much better compared to what the PlayStation 3 is capable of.
A teaser for III at the end of the 2.5 trailer released before E3 2014 got much hype...except right after Square said III wouldn't show anything more at E3 2014, practically crushing the idea that fans would get more of an explanation of what the teaser was saying, or anything else new. This wasn't taken well by quite a few people.
The D23 expo trailer not being shown. Square or Disney seemed very determined to not show the trailer for whatever reason. To say that this caused a bit of ruckus is quite the understatement. Somewhat sated when the trailer dropped at Jump Festa...literally over a month after the D23 event. No one will likely let Square forget the lack of communication here.
The fact that Square is still hiring people to develop the game and that the team developing the Final Fantasy VII Remake got people from the Kingdom Hearts III development team to help them didn't do people in the fandom any favors—especially considering that Square previously stated that once development on Final Fantasy XV wrapped up, all resources would go to this game.
As of March 20th 2017, the awaited announcement that was dropped "This Winter" has proved underwhelming, consisting of a few screenshots of Sora fighting a midlevel boss called a "Rock Troll" from the already announced Olympus Coliseum world (the confirmed first world in game) and some clarification on his new forms/keyblade uses. There is also the 2.8 secret ending, which boils to them going "We SWEAR this is the last bit before KH 3. Look, Sora is going off to Olympus Coliseum right now." (It's a lie, KHX just revealed that it is going to be critical).
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 is in development, but the first full E3 trailer had some awkward looking facial designs.