The ending of Mass Effect 3 isn't as horrible as people are making it out to be, but it certainly is a bit of a curveball. It's pretty clear that the ending is meant to be thought-provoking and polarizing, a case of where the only answer is the player's own interpretation. The lack of closure only enhances this effect, Bioware deliberately denied us an answer so that we can find one for ourselves. Bioware gave Mass Effect the ending people needed, not the one they wanted. Bioware gave Mass Effect the ending players needed, not the one they wanted. Or something.
Understandably, some people aren't taking this well.
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If I were to compare the ending to something else in media, it would be Neon Genesis Evangelion. Structurally in a sense, perhaps even thematically, absolutely in terms of how fans are reacting to it. The endings to NGE and ME3 both lack closure, but both are powerful and unforgettable.
Love it or hate it, there's no denying the ending to Mass Effect 3 made you feel something. It could be rage against the creators, confusion, or strange sort sumblime bliss from going all meta on it.
The players are left to decide what the ending means... for themselves.