One of the most important things is usually the game's mechanics, particularly the shooting and movement mechanics. This includes things like the accuracy and recoil of the guns, the speed and responsiveness of the player's movement, and the overall balance and feel of the gameplay.
Additionally, the design of the maps and levels can also be critical in FPS games, as they can heavily impact the pacing, strategy, and overall enjoyment of the gameplay. The level design should be intuitive, offering a variety of paths and areas to explore, while also encouraging players to engage in combat.
Finally, multiplayer functionality can also be essential in FPS games, allowing players to compete and cooperate with each other in various game modes. Good netcode, server stability, and matchmaking systems can all contribute to a smooth and enjoyable multiplayer experience.