Not to spark a debate exactly but I am viewing it, I think, from a significantly different lense, at least with regards to the non-bender / bender dropped plot line.

The representation of disenfranchised minorities in fictional narrative is, to me, incredibly important, being a part of intersecting minority groups (POC, queer, female-bodied, non-Christian); it was incredibly important to me to see how they would handle — if they would handle — the subject of people who are systemically oppressed with care and with awareness. They did not, and it was hurtful on a personal level because things like that affect me on a daily basis, and it was painful to see something that promised to tackle those issues head on go instead by way of just furthering the oppression of those people (having Amon be a bender himself and taking advantage of the plight of the disenfranchised non-benders), and not addressing those issues in favour of a very poorly portrayed "romance" subplot.

With regards to Korra not learning airbending like they said she needed to, I think my gripe with that is less about the failed plot (though that is a part of it) and more how her character development suffered to make way for Mako angst. She didn't grow as a character at all in the first series. The part that was supposed to make her grow was just given to her circumstantially. Aang had to learn how to get over his firebending fears; Korra happened to be able to bend because her ~boyfriend~ was in danger. There were very few consequences for Korra for her actions — not personally / socially, and not as part of the systemic oppression I described above (The line "You're OUR Avatar too!" during the non-bender round up seemed to be ignored after that one instance).

I don't think it's like Mass Effect at all. The ending of ME3 (which let's not discuss here ;P) wasn't great, no, but it wasn't just the ending I had a problem with in Korra, it was literally the whole plot and romance subplot and how things failed to live up to their promise and premise.

For clarity though, I don't hate the show, of course not. I wouldn't be here if I did. But those are the reasons for my apprehension of the new Book. And, because I know the above takedown is pretty harsh, I want to remind everyone that critiquing something harshly does not mean that you love it any less (sometimes, the harshest critiques are born out of disappointment in a thing we hold dear).