In boardgames, there are (for me) three main aspects that determine how much I like a game. These are Theme, Flavour and Mechanics.
The Theme is the overall story or setting of the game. It could be a Star Wars game or a Cthulhu game, a Sci-Fi space exploration game or a Sword and Sorcery D&D game. I am less likely to play a game if I am not fond of the theme. I don’t want to run a farm, for example, or sell textiles in medieval Florence. I want to shoot stormtroopers, or fight dragons, or try and stop the world being eaten by oceanic elder gods. You get the idea.
Let us suppose (correctly) that I am interested in any game that has anything to do with Cthulhu. What makes a good Cthulhu game?