Let me just say, I was a fan of Avatar: The Last Airbender (mostly Earth and Fire) but Korra tops it without a doubt. The writers and creators have really grown and learned their art, and it shines through in this series. It's darker than it's predecessor, and the story is much more intriguing.
The main antagonist is Amon, a patient, cunning, and ruthless (supposed) non-bender that is leading a rebellion against the capital Republic City. His plot starts out as no more than an intriguing idea, but in the scenes where it shows him committing his deeds, they seem horrible, unspeakable and more than devilish. It pulls at the watcher more than many death scenes that have been produced.
Part of that is because of the empathy the viewer will quickly grow for the extremely well- written characters. Each character is starkly different, and each one has a past that comes into play in the story, some much more than others.
The main sell point for me on the series is the magnitude of the events. I've watched many T.V. shows and read many stories that had to try those great "OH SH-T" moments, but LoK has me actually shouting it so loud I wake my family, even when I can predict what has happened.