Rules

Like many RPGs, one of Mass Effect's primary selling points is its freedom of choice, which translates to freedom from certain rules that may constrain more linear games. The player has the option to select how Commander Shepard will look for the duration of the game, what his/her back story is, and how he/she will react to situations, verbally or physically. The player is not even restricted to the world of the story. Even though the mission to stop Saren is the core part of the game, there are other planets the player can visit if he/she chooses. A strong effort is made to make the player feel as though they are in control of their destiny and that what he/she is doing is meaningful. The rules in Mass Effect are not based on achieving a high score or defeating another human player through mental or physical means, but rather accomplishing various objectives to eventually solve a mystery and defeat a villain. The goal is to have an emotional experience with the core story and the exploration of the galaxy.

1. Operational Rules: While there are a few sequences which must be completed quickly, defusing explosives before the timer runs out for example, Mass Effect has no overall time limit. Commander Shepard is free to go around the galaxy at his leisure. The only thing compelling the player to finish the game is the emotional impact the game has. Of course, the galaxy won't really end if the player doesn't stay up late to beat the last level instead of taking care of it in the morning. Within the game, the physical rules are realistic to our universe with certain exceptions, such as the common inaccuracy of sound being able to travel through the vacuum of deep space when ships fly through it (though the dogfights would be a lot less dramatic in silence). Also, the convenient discovery of Prothean technology and biotic powers allows the weapons, vehicles, abilities, and especially locations to be literally out of this world. Other than that, though, physics [attempt to] behave realistically. Gravity holds you to surfaces and environmental factors like lava incinerate you while you explore these surfaces, just like in real life! Sadly, realistic rules do not always apply to the NPCs. The civilians standing around cannot be fired upon and if you want to shut Liara up, you'll need to find a way past her invincible shield that absorbs every projectile you throw at her. Programming outcomes for these actions into the game would be nearly impossible though. The possibilities would be endless. The consequences that have been put into the game involve certain gameplay penalties or rewards and what emotions the player feels in response the situations that arise, the NPCs that die, the relationships that form, the musical score, and the gameplay itself. It is difficult to quantify these and they will be different for every player. On the other hand, more standard processes, like the death of Commander Shepard, will probably happen for everyone. If he/she absorbs enough high velocity particles to drain your shield and life bar, the Commander will die. This is only temporary of course. The player has the power to travel back through time using the Save and Load features and can try sections of the game until he/she can complete them.
Sources of damage for Commander Shepard come from the assault weapons of his antagonists, melee attacks from less sophisticated creatures, or even a suicidal grenade thrown by the player. On the other hand, if you make your enemies absorb enough high velocity particles to kill them all, and can manage to find solutions to the fairly simple puzzles on each planet, you will eventually win.

2. Constitutive Rules: Reducing Mass Effect to its most basic mathematical structure is not recommended because it severely cuts down on the immersion you feel. When the player becomes aware that they are playing a game, the constitutive rules, the underlying structures that make regulate the game, are noticeable. Even though conversations are a big improvement over Elder Scrolls IV: Oblivion and KotOR, you can still tell they are scripted. The construction beneath the story are fairly visible whenever a line of dialogue or a motion (like the fist into the palm) is repeated. Other obvious repetition can be found on most of the side quest planets. They are mostly just very similar mountainous regions with different textures and environmental effects pasted over them.
Using elevators to get between floors is another annoyance that is necessary for the game to disguise load times. It is preferable to a blank screen, especially when an interesting dialogue accompanies the ride, but there are just too many of them, especially on Noveria, where there are also no news flashes or conversations between your party members. Some of the rules exist to keep the game balanced and functional. For example, it might be prudent to take all of your allies and the marines from the Normandy with you to help in your battles but you can only have two NPCs with you at a time. Each character has his/her own skill so it is up to the player to decide how to balance his/her squad for maximum combat effectiveness.

3. Implicit Rules: Any of the gaming standards that Mass Effect conforms to could be considered "implicit rules." The basic controls, for example, should be familiar to most console gamers. They are written in the instruction manual and should the player need to refer to this, the control scheme would likely fall under "operational rules." It is also implicit that your actions will have consequences. The game is basically a life simulation, even though the life is outlandish and in extreme circumstances, and in life, actions have reactions.