A nice well told story, a well balanced and well paced action gameplay, vibrant colourful visuals, a devoted publisher with commitment to the fanbase, good weapons, cool alien races that are shown to have human personality (only if it DOES have aliens), some exploration and atmosphere amazing epic music, fun multiplayer and that wierd X factor of charm that is hard to describe that stems from happy memories of the past or even just an established familiarity or even just spme unknown source of charm.
Also preferrably NOT set in the modern world, the distant future or an underwater city or a distant planet or city infested with monsters much more prefferable.
Also OP, welcome to the escapist... though I have issues with your choice of username.