There's no absolutes here. Everybody has different tastes and different things they deem "fun". In my opinion its easier to put a finger on what makes a bad game, and try to avoid that.
In my humble opinion however:
Variety, but not in excess is important. Too little variety there's little motivation to keep going. Overdo it however and there's never a sense of cohesion.
Learning curve. Give a player a level or 2 to come to grips with the game controls and basic mechanics. Again, dont overdo it though, or a person will just start thinking, "enough already, I just wanna get on with the game".
Im also a fan of a good storyline. Doesnt have to be the best story, but something engaging for me helps me me want to continue a game to see how the story pans out.
Audio. Especially for a dark, atmospheric FPS shouldnt be underestimated. For A grade modern gaming this atmosphere can to a degree be created with visuals, but for older games/game engines sound adds more. We have natural sensory responses to audiovisuals, so try to have at least one or the other adding to atmosphere.