The Onion AV Club has an interesting take on the "happy" ending.
But watching the end again, this way of reading the last five minutes popped into my head, and it makes more and more sense the more I think about it. Korra’s lack of identity and her block were actually the same thing, and they’re both resolved in a kind of great under-the-censor’s-radar way.
Korra’s just found out the best healer in the world can’t do anything for her. Her last hope to hang onto the identity she’s had since childhood is gone. Mako tells her he loves her; she tells him to go away, that she’s “not the Avatar anymore.” Her misery calls back to the dream she had in “The Voice In The Night,” where her own subconscious—dressed as scary Amon—tells her “Once I take your bending away, you will be nothing.” Korra goes out to a cliff, and walks up to the very, very edge—she’s close enough that from her looking-down POV we see a tear actually falling all the way down the side of the cliff, which means her head is leaning out over the drop.
I can’t think of a reason to use that specific shot unless it’s to imply that she went up there to throw herself off the cliff.
Supporting this theory: When she pulls back, sits down, decides to keep living as a person who isn’t inherently special, and starts thinking for real about what that means—that’s the instant Aang finally shows up to declare “You have finally connected with your spiritual self!” This all feels like fan-wanking, but the alternate explanation seems to be “He showed up then because she was… continuing to be sad? And that continuing sadness somehow resolved her spiritual block?” That’s unsatisfying, unearned, and also doesn’t really make a lot of sense. The more I think about the oddness of that POV shot, Aang’s timing, and, most importantly, how not okay Nickelodeon would be with showing a hero contemplating suicide, the more sense it makes to consider the final-final battle of the season as completely internal—and pretty damn dark.