These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The game being announced on Xbox One had a lot of people crying out in disgust. For some, it being on Xbox One isn't so bad. It not also being on Wii U (considering how many of the side games have been on Nintendo handhelds) is the real complaint.
The complaints about it being on Xbox One have died down somewhat after it was revealed PS4 players will be getting exclusive content.
Also, 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).
The absence of the game on the Wii U is perhaps justified if the game is using the same development method as Final Fantasy XV, i.e. developed primarily for DirectX 11 and then ported to consoles that support it. What the Wii U's architecture is actually capable of supporting has never been fully made clear, but it seems to be something between DirectX 9 and 10 in terms of power. However, certain indie developers (like the ones behind indie platformer Candle) have stated that it is indeed possible to get Direct X 11 level graphics on the Wii U with the right techniques, causing a select few to wonder if Square could perhaps just be making excuses similar to EA's infamous excuses relating to the Frostbite 3.0 engine. Combine with the Internet Backdraft relating to the game's graphics, and some are wondering why the game is not on Wii U despite not looking much better than Wii U offerings such as Super Mario 3D World and Bayonetta 2.
After ownership by Disney, the possibility of Marvel and Star Wars being represented in the game at all has become a fairly divisive topic. Even more so since Nomura himself stated he wished for their inclusion.
That being said, it's fairly common to see fans that want a Darth Vader Bonus Boss - even for people that don't want to see a Star Wars world in the game.
Another sour grape for half the fans is revolved around the gameplay. It uses the traditional command menu system like in the first Kingdom Hearts and Kingdom Hearts II as opposed to the command deck introduced in Birth by Sleep. Nomura himself stated that the command deck was a natural evolution of the system, thus going back to the menu interface after using the command deck for three games in a row comes off as suspect in the eyes of some.