Mass Effect isn't so much about the consequences of your decisions as it is about the player making those decisions. By extension, it's a metaphor for life; your actions and decisions shape who you are. You can always look back and say whether or not it was the right decision to make, but at the time you simply have no idea what the consequences are. Sometimes your options are all unpleasant, but you have to make a decision just the same or else nothing good will come of it all.
At the end of it all, you are give the power of a God.
You are given the final decision which will determine the fate of the galaxy.
You have no idea what the full consequences will be, and you never will.
But you are still given the responsibility to make that final choice.
What is your decision?
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Essentially, Mass Effect is about self-determination.
The nature of the final decision and the lack of closure are to emphasize that we live in the present, not the future. We can always weigh our options based on what we feel will give the best outcome, but the truth is that we never know what will genuinely happen. So have no other option but to take life as it comes, choosing our actions every step of the way. In the end, we not get what we wanted... but we made those decisions just the same.
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When I was faced with that final decision, my thoughts were:
"I hope I'm doing the right thing..."
By extension, these were (my) Shepard's thoughts as well. All that preparation, only to be faced with something else entirely. Nevertheless, my motives were do what I felt what was best for everyone. I won't know if was the best decision, or even if there was a good one.... but what I do know is I tried to do what was right.
For those who are outraged by the ending, it would be safe to say (their) Shepard is outraged as well. Having a choice thrust upon them, being denied the (happy) ending they so wanted. But it's what you get.
In the end, what you (and by extension, Shepard) feel about the ending... is just that.
No more, no less.