BioWare created a monster.

Mass Effect is a series with unlimited potential, granted, it's not the first RPG where your decisions affect the world, but it plays with the notion that everything you does will come back to bite you in the ass sooner or later, and it did, for most of the game, characters wouldn't forget your kindness (or lack of), sometimes I even found myself asking who the hell was that person sending me an e-mail thanking me for saving them. Feedback, all the time, you could influence your companions through your own personality, handle people and situations in so many different ways I wouldn't doubt that there isn't a single Shepard with the same history.

The game's narrative style was so advanced and became something so big, that when it came to the ending BioWare couldn't live up to it's quality, maybe it's a flaw from doing something SO RIGHT, with few flaws here and there that anyone would overlook for the sake of the overall quality, that the mechanics of the game would make it impossible for the ending to live up for that quality. There are so many variables, from that colonist you spared in Zhu's Grace to the amount of Cerberus soldiers you killed (not really - but still, variables), that maybe BioWare decided to go for the 'simple' solution and gave us a 'push this button for fireworks' ending, no effort.

The bad taste of defeat was that there was nothing to defeat, I thought the reaper battles through the game were preparing us for THE battle, fighting six Brutes and another four Banshees wasn't all that pleasant, but they weren't bosses. The defeat was the disregard for all the work Shepard had, during all those games, was because rallying the galaxy to a single objective wasn't something positive in the end (actually, taking in consideration all the military fleets are stranded on earth, turned it into a rather negative point), because all you had to do was push a button.

Mass Effect became something so innovative, and so huge, that BioWare simply lost control over it, the gamers had control, there was no Canon story, they tried to pull an Elephant (the entire ME story) through the eye of an needle (the endings), narrowing every experience in this huge and variable universe, in three different 'choices', destroying many Shepards moral grounds, having them to choose between choices they would have never accepted.

Maybe the sad truth is that there wouldn't be a good (by that I mean with quality) ending, regardless of how much effort BioWare had put on it (though I don't want to believe that, it's not excuse for their horrible ending either), because there is no game mechanics able to deal with that amount of variable and expectations, because no ending would possibly live up to the series' quality.