Final Fantasy XV is a very good game. But fans were naturally vocal about their opinions with some praising the gameplay and others criticizing it for the lack of characters and story.
Whatever your feelings may be you should totally let Square Enix know about it. A recent survey has popped up that focuses just on FF XV. Plenty of the questions are about you as a gamer but towards the end the more serious hard hitting questions start to appear such as ‘What would you tell the developers of FFXV?’
From my front I wished they added more areas to explore primarily Insomnia which was heavily promoted in the past. Extra story and character would have went a long way as well. But most of all they should not have spoiled so much with the trailers. Anything before the linear section gets heavily shown with some trailers even showing parts of the ending.