Complaint and criticism is one thing, but what we're getting a lot of nowadays goes way beyond that. The problem is that seemingly a lot of people are missing the point that satisfaction with a creative work isn't something that can honestly be guaranteed under any circumstances. Its an intention at best.
Ending long-running sagas is a fraught process, hell off the top of my head The Sopranos, Battlestar Galactica, and Lost all had extremely divisive finales yet none were the subject of silly "retake" campaigns. I say silly because "retaking" implies that authorship was once in the hands of the consumer, not Bioware, and that was never the case.
The difference between the examples you cited and Mass Effect 3 is that the ME series touts
player agency as a major part of the experience. This isn't a story you're watching, it's one that you're interacting with - you're the invisible hand guiding the experience, directing the protagonist's actions, shaping his/her relationships with the other characters in the world, and guiding things toward outcomes that you, the player, desire. So there's definitely an element of 'authorship' present in the experience. While it's not realistic to expect that every possible ending should be accounted for within the scope of the game, there
should be an element of meaningful choice there, and one or more of those choices
should be one that's going to feel satisfying for the player.
The ending in a game is a
goal to be achieved. It's a payoff for the
player. That's particularly true of a series where players have guided the same character through three successive games. They're invested in the protagonist in a way that you just don't see in other media, because they actually had to work to get the character to that point. It's
their Shepard. And many people seem to feel (and I'm inclined to agree) that Bioware fumbled the ending.
Another point to consider is that we live in an age where publishers can address any number of issues in a game through post-release patches. This differentiates videogames from books or novels, which are more or less frozen in amber when they're released (at least until the inevitable remakes roll around...) If gameplay bugs that reduce a player's enjoyment can be addressed in patches, why shouldn't something plot-related receive similar attention in a game of this type? The ending as written is considered 'broken' by the vast majority of fans, and it's hardly a case where they're just too shortsighted to appreciate its artistic brilliance. It is genuinely lacking. It's impairing peoples' enjoyment in a major way. The technical means to address the problem are there. If publishers can patch in advertisements on load screens after a game's released, what is wrong with users campaigning for changes to a product that they actually
want?