Many players said that the complex fighting strategy that the old fight system provided in earlier final fantasy games was much more interesting than the real time battles that lacked a true strategy.
In earlier Final Fantasy games, each character had a very unique identity. In Final Fantasy XV, characters were introduced so fast that they were hardly memorable.
The developers have tried to please new comers, and as a result, they moved away from many of the old game elements that used to appeal to hardcore fans. The result was a lost identity and a game that doesn't feel like Final Fantasy games. See why Final Fantasy 7 was great.
In Final Fantasy XV, side quests interfere with the main story from the very beginning of the game even when the person is still being introduced to the story. Some players described the side quests as distracting and boring.
Many players have complained about the story and said that it was weak or not compelling. Final Fantasy hardcore fans always expect a breathtaking story and this is why some people got disappointment by Final Fantasy XV. See why many people liked Final Fantasy IX.
Some fans criticized the game for having unlikable characters. Some players felt unable to identify with or connect with the main characters of the game, and as a result, they felt disconnected.
In many of the successful Final Fantasy games such as Final Fantasy VII, the characters were introduced properly with strong background information given about them in such a way that people become truly connected with them. This process didn't happen properly in Final Fantasy XV.
Many people didn't like the idea of having to move around with the car all the time. This made the game seem like a bizarre combination of different games put together.
Many players complained about the Camera angels during the fight and said that it made the experience unpleasant. See also why Final Fantasy games are too long.
Some people said that the game was repetitive and boring. The player is assigned the mission, uses the car to locate it, grabs something then starts over.
Some players said that the game feels weird, as they failed to recognize whether it's an open world game, a road trip game or a Final Fantasy game.