A) These are all movie examples, not games. Completely different medium of entertainment that you can't make comparisons to. For one thing, movies and books don't have gameplay elements to them. Books don't have vocal elements to them, and movies don't have interactive elements to them. There's multiple differences between the two, enough so that a comparison doesn't really work.
There is no difference between the plots of video games and movies other than player interactivity (which, by the way, is a central complaint of the ME3 ending: the player's choices were ultimately meaningless). What, are you going to argue that we can't compare television and movies now?
B) These are gross exaggerations as well. Mass Effect 3's ending doesn't do this. Maybe it's underwhelming and maybe it contradicts prior events, but there's no musical number breaking out at the end of it.
I've never seen outcry like this before, so, shockingly, it may not be that much of an exaggeration.
And the overwhelming majority of video game RPGs have competent endings. Not always great, but satisfactory.
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The Many Forms of Kobe:
http://i39.tinypic.com/oqx98z.jpg -- credit to KIRBEH