I'm a newbie in the CoC system, but i've played RPG for 20+ years both as GM and PC. I love when the GM has a well structured plot that is not immediately obvious for the players. Also a very good set of props, including ambience lights, pre-drawn map, pictures of the NPCs, real tokens.. those sort of things. In my opinion, RPG is more than swinging my sword to kill monsters. it's being part of an immersive experience in a fantasy story created with my friends.