The impression I get from everything I've read (The situation interesting me far more than the abusively DRMed games! :D) is that Bioware wanted to create a unique and unexpected ending to the series, but unfortunately they didn't really know how to do it, so went about it the wrong ways.
There are so many obvious missteps, from the sudden inclusion of multi-player in the third game of a single player series to promising 16 endings and delivering three. Worse, those three don't seem to have any relationship with anything the player has ever done throughout the rest of the series.
To me, the three most obvious endings in anything like ME would be 'win', 'stalemate' and 'lose', as it is basically about a war. But there's none of that. For more endings, you could graduate from 'total win' to 'total loss' with many shades between. Those are things that would make sense.
But to end a three game series where everything till the end was predicated on player choice, then take that choice away at the point of climax is... An unusual decision, shall we say!
Everything you've done through all three games up to that point is rendered completely meaningless. I can't think I'd be happy with that, either...