Excellent post (as always!), and I have only a few minutes for a response, so you'll excuse if I just whole-hog agree with a lot of what you say.
For me the big "thing" about the Mass Effect series was the (sadly, illusion) of consequence. Not that I don't agree that shackling their morality to the "Light Side/Dark Side" binary was horribly limiting (Save the kittens, or throw them int he furnace to power the ship? Really, those are my only two options?), but because even with a well-written binary of decisions, they promised consequences.
Do you save the Racni? Spare the Council, or spare the Human Fleet? Keep the Collector Base, or blow it up? There are a dozen really significant decisions you make through ME1 and ME2.
And the ME3... doesn't care. "Oh, you saved the Council? Okay, you get +20 points towards... nothing, really. But if you saved the Human Fleet, you would get +20 points towards nothing!"
Like... oh my gods... the irrelevance of ALL MY CHOICES.
And the ending. For anyone who hasn't yet enjoyed the game (and Mass Effect 3 is, in my opinion, a tour-de-force of an RPG right up until those last 10 minutes)...
The main narrative thrust of Mass Effect is that Shepard resists. It's what she does. Overwhelming odds? Fight. Hopeless mission? Fight. Thousands of unstoppable AIs? FIGHT. And then you meet the MacGuffin-bad guy, who shows up in the LAST GODDAMN 5 MINUTES, and Shepard just... "Oh, I'll passively pick one of these four things, and NONE OF THEM RESIST. Even the "shoot the MacGuffin bad-guy", which SHOULD be the right answer, results in Shepard dying and everyone losing. NO. Just no. I felt goddamn betrayed by that ending. It was lazy, but I can forgive lazy, but what I can't forgive is that it flies contrary to THREE GODDAMN games about fighting impossible odds and just says "and in this case, you don't fight. The end!"
The only defense I've heard, somewhere, is that Bioware actually had a much better ending planned, and that the ending we got is actually only the half-way point of the game (Shepard has been corrupted by the Reapers, and the "final" scene we see is actually an elaborate mind-game-trick that leads up to ACTUALLY kicking the Reaper's butts)... and then ran out of money. Which is still pretty dang near unforgivable, because they could've cleaned up their mess before they just cut the story... but it at least makes my have some sympathy for them.
We will see about ME:A. I was SO MAD about ME3. I'm not sure I'm willing to give them the benefit of the doubt.