No need for spoilers;
Mass Effect allows you, in Mass Effect 1, to chose one of three pre-determined backgrounds for Commander Shepard. You choose if Shepard is a man or a woman, where they come from in the universe (out of three more choices), and what they look like. Through a series of dialogue choices you make, you guide Shepard's ideaology between SPACE BADASS and SPACE GOODCOP. Some of these choices are small, and some of them are impactful up to and including party character deaths.
Mass Effect 2 and 3 allow you to import saved games, retaining your Shepard's backgrounds (which comes up in conversations regarding your history), looks, and prominent choices. You are also able to romance characters, and since some of these characters return in future installments, you can carry on a lasting relationship with them.
All of this causes the player to form a deep bond with Their Command Shepard, guided by their choices and actions and decisions across three games and five years.
The final twenty minutes of the game are the same for everyone regardless of the choices they have made along the way, save for absolutely minute variations in the ending cinematics which are changed only by flipping one of three switches. The actions you have made up to this point do not preclude you from flipping any of the switches (though one only appears if you do enough sidequests.) This lack of acknowledgement and player input after the investment of 30 to 100 hours of gameplay and five years of attachment has lead to an absolutely monumental shitstorm the likes of which has not been seen since the Star Wars prequel trilogy.
