I will preface this by saying I barely didn't make it through the first season of this series, stopping at the last episode.
I'm biased from watching original series, which was way better. When comparing the two side by side, there are canonical inconsistencies that they make no attempt at explaining - a missed opportunity for interesting story-telling and relating back to the original series. Where did all the new sky bison come from? How did blood bending survive from the original series? Why is it that Korra is "opposite" an air-bender even though she is from a water tribe, wouldn't the hardest element for her to learn be fire, based on the original series' explanation of the elemental relationships? Seems like they just used "Air bending is hard for me" as a filler in the writing process, forcing interaction and conflict between Aang's son and Korra. Wouldn't it make more sense for Korra to be born to an earth-bending family, if she was going to have difficulty learning air-bending?
It's also convenient that Korra is born a prodigy who doesn't need to be taught forms or proper movements to bend any element (except air), which the writers try to set up as a humorous moment but merely butcher the nuance and essence of the show's juice - bending being a difficult-to-master martial art form butchered into something anyone can do by chance.
The show's identity is boring. The plot is extremely droll. The creators transitioned from an action/adventure with a fairly open story-telling style, one in which it felt as though anything could happen with each new episode, to one in which it seems like a mediocre mystery/investigative style.
The characters are undeveloped. It's really sad that they decided to age or retire so many of the characters they painstakingly (and masterfully) developed in the previous season in favor of new characters that are stiff and boring. Any attempt at fleshing these characters out is cut short by the introduction of new boring characters.
Many of these characters can be explained with very little effort, which is actually what they do for some of them. For instance, in one of the first episodes, Tenzen is described as someone who is always "so serious." Which is hilariously his whole identity - a serious politician whose dad was the Avatar. Tenzen has a few kids who do very little for the plot, or even for themselves developmentally. Then, there's a love triangle with an oafish earth-bender, his pretty-boy fire-bending brother, and Korra, which transitions into a love triangle minus the oafish earth-bender plus a non-bending chick whose contributions to the show are that she is rich and hot and knows martial arts. Toph's daughter is in the show, as the metal-bending chief-of-police, who favors using a metal wire attached to her wrist to throwing rocks, when the latter would remedy situations much better than the former at times.
The show's not good. It's inconsistent with itself and untrue to its original identity. Conflict here is forced and plot-devices are conjured out of thin-air.
I can't go on. I'm beginning to rant. It's actually painful to continue to write. I could go back to the show for more references and specific examples, but then I would have to sit through more of that disappointment and failure.
It's not that great. You don't need to watch it, even if you are a huge fan of The Last Airbender. You will be disappointed if you are, actually. Or you will lie to yourself because it's easier than accepting the truth.