It can be said by many gamers that the most important part of a game is the fictional world and the story behind it. Juul seems to agree in Chapter 5. This is not to say that rules are not important; rules and fiction go hand in hand. However, without a good story, it becomes difficult to draw players in. Some games (such as Mass Effect) have been reported to have too much story. Others (such as Call of Duty) have been said to have no story. I am no expert in the Call of Duty franchise, but there does seem to be more of an emphasis on just shooting other people with no real objective. Meanwhile, in my opinion, Mass Effect has created a fictional world that draws people in. Complete with history, politics, and even religion, the three games have managed to create a world that has been dreamed of since humans first set their eyes on the stars.
Mass Effect shows us a galaxy set in 2183. Due to the discovery of ruins from an advanced civilization on Mars, humanity has managed to advance technology at a beyond rapid pace. After the discovery of the mass effect, the manipulation of the newly discovered element zero with electrical charges that is capable of manipulating space and time which allows for faster than light (FTL) speeds in spacecraft, humans moved further into their own star system. After discovering the mass relays, a network of devices designed to allow FTL travel all over the galaxy. After foolishly activating the first relay found, humanity found itself at war with the militaristic and rigid turians. This war, called the First Contact War, introduced humanity to the galactic community. After finding it's place with the races of the galaxy, a chance mission on the human colony of Eden Prime leads to the discovery of a race of machines called Reapers that every 50,000 years cleanse the galaxy of all space faring life. Only a handful of people believe Commander Shepard's claim that the Reapers are returning. After a chase all over the galaxy, one of the Reapers (Sovereign) attacks the Citadel, the center of galactic government. However, the Council (the central governmental body) still refuses to believe the Reapers are returning.
Sending Shepard on a clean up mission, Shepard's ship is attacked by an unknown enemy. Shepard is killed in the attack. However, the terrorist organization known as Cerberus brings him back with advanced technology, luck, two years, and billions of credits. The organizations's enigmatic leader, The Illusive Man, sends Shepard on a mission to discover why human colonies are vanishing without a trace. Shepard is sent on a mission to recruit a team of specialists to take down the race responsible (the Collectors) and tracks them to their home world to destroy them. Depending on the preparations for this suicide mission are taken, the mission can be a total success with no casualties or a disaster with even Shepard himself dying. Regardless of the outcome, the Normandy cuts all ties with Cerberus and the Reapers are seen just outside of the galactic rim.
Shepard is sent to Earth in 2186 to answer for his partnership with Cerberus. During this, humanity's fleet admiral Steven Hackett reports that something enormous is coming for Earth and mobilizes the fleets. Despite these measures, the Reapers cut through their defenses and lay Earth under siege. Shepard is sent to get allies for the fight, but is ordered to go to Mars for the blueprints of a device that could defeat the Reapers. After finding the plans, Shepard learns that Cerberus is now the enemy, with the Illusive Man planning to control the Reapers for human dominance. After getting the blueprints, Shepard begins to recruit allies to built the device (the Crucible) but also to protect it as Earth is to be retaken. After securing alliances most would say impossible, Shepard leads the assault to retake Earth and depending on the choices made, the galaxy will change in several ways.
This fiction has drawn players in and has helped to create a world that seems eerily possible. This excellent writing is possibly the best way to fully appreciate how important fiction is to a game and how it can make it or break it.