The Mass Effect franchise—if played for reasons other than just killing aliens—is an exercise in scenario visualization. Faced with the possible extinction of the entire galaxy, Commander Shepard is called upon to act in an apocalyptic circumstance the proportions of which cannot be visualized--even the imagination strains to conceptualize all organic life winking out. The game toggles deftly between the various heartstrings that must be pulled to wed you to this effort—from the individual to the social, the game excels at presenting different scales of attachment that help you stretch your empathy outward, unfurl a hope for a future that motivates your action.
Closest to the level of the individual, you have your lover who provides an emotional foil for Shepard's inner thoughts. Yet your investments unfold beyond your romantic dyad, from the crew, each with their private demons, to the people of the Citadel who need your aid in both personal requests and species-based dilemmas, to whole planets, fleets, and biological ecosystems. You understand that your actions are consequential, always moving outward, and you are asked to consider the potential consequences of those decisions, how those choices alter the fragile scenario that you are trying to stabilize. Who will help you, and who will turn into an Achilles heel? How can you always work toward the best possible outcome for the largest possible beings? What guides your ethics—self-preservation, a rigid code of honor, case-by-case pragmatics? A good gamer, a mindful gamer, is thoughtful about these decisions, and recons with them.
The grand failure of ME3's ending is that the power of what the platform was able to do ceased to matter. Suddenly Shepard is dropped into a situation over which he/she has no ability to foresee. The grand universe scenario that you've been visualizing, managing and calibrating doesn't matter. All your careful planning, all your thoughtful choices, bottom out to a numbers game, a hollow stand-in for the intellectual and ethical work and exploration we filled Shepard up with for 90+ hours over the course of the trilogy. The game works powerfully as a simulation of leadership development, a playground for trying out responses on how to treat others, a way of thinking from the micro to the macro, and how these scales of behavior are latched into each other--how this thing over here effects that thing over there. That's the reality of living in our world—that our actions have consequences that are always spreading out beyond us, slipping away from us, operating in systems of power we can't totally comprehend. Complexity. That's some of what Mass Effect got at. That's why it felt particularly alive. When that complexity is removed, when suddenly the whole world is just the machination of a never-before-seen godchild/deus ex machina, we've traded a complex systems model for a top-down hierarchy model. It's this flip, wherein we are removed from meaningful choice--which was in no way keeping with the structure of the game--that feels most insulting, saddening, and disingenuous to a game that helped me imagine what I might want from a future I could cooperate in.