it really is as simple as being able to distinguish opinions from facts, and clarifying each in the revew.
Facts: This game has X combat, Y selectable characters, the crafting looks like Z, etc.
Opinons: This game is amazing! 10/10! Best story of all time! GOTY!, etc
You absolutely can have an objective game review, it's just that no writer wants to do that. They'd rather make it more about their opinions of the game than of the game.