Referring to this game specifically, probably that fact that it was turned into this game about idols instead of what could (and should in my opinion) have been. It's overused when it starts getting into franchises that it really doesn't fit in the first place. Yes, I'm aware the developer can do whatever the hell they want with their franchises. That doesn't mean I have to like it. Nintendo ruined Samus's character with Other M for instance.
Other than this game, there's been the things about Fire Emblem Awakening and Fates having the more "otaku-aimed" character designs rather than what FE was in the past. It's something that irritated me in awakening, especially the redrawings of characters for the DLC, as many of them looked awful. Micaiah was one of the biggest offenders there. And of course I'm sure everyone has heard all about Nowi and the problems there. I acutally enjoyed awakening, but I think if I would have enjoyed it more if those things were not in the game.
I've been pretty outspoken about this before, so I'll try not to go too far into a derail here.
I noticed this in the FE World video:
It's a reference to the demon Ame no Uzume from Shin Megami Tensei:
Looking at the enemies, it doesn't look like we will see SMT demons as them. I do hope they at least don't go the Persona 3/4 route and start having enemies be palette swapped colors of weaker enemies though. Dungeons looked kind of cool though.