This post is going to have spoilers for the current series of Legend of Korra proceed at your own risk.
I love Avatar the Last Airbender. There are so many good things about it, I've blogged about it before. One thing that the writers have not shied away from is having an arc of growth for their characters. Aang's arc was maturing from a child to an adult (at least in so far as responsibilities go) and learning to act decisively. In the new series the current avatar Korra has the opposite problem. She acts decisively but with out thinking or planning. She has no contact with the spiritual side of bending but she has been able to bend every element but air from a very young age, younger then her predecessor.
Just that simple choice of flaws helps radically define her character and make it clear that this is her story and not a continuation of Aangs. Throughout the series a character behind a mask by the name of Amon has been threatening benders in Republic city. There has been a lot of speculation as too who he/she really is under the mask.
I am finding as I look at this series through the eyes of a writer (mired in one novel's revisions and another first drafting) that I am guessing some of the plots trajectory. You don't put a character under a mask unless removing it would reveal to much information to the people. That's led to some speculation that Amon is one f the characters from the original series, that he was Tarrlock, that he was Tenzin, and many more. My pet theory is that he is a spirit but we'll see how that speculation plays out. Another theory that was brought out was that the avatar was split into to beings, the physical side with Korra and the spiritual with Amon.
There's one problem with that theory. Korra would have no growth arc. The logical
progression to that kind of arc is some sort of melding and it means
Korra doesn't have to grow and change
and learn spirituality. So far she seems to be changing. She's
had contact with Aang's past life last episode and she's starting to
learn patience. I am going to stick with my spirit theory.