I'm sure watching it is not "necessary" in order to understand the story in this show, but it was a great series, and a few of the characters we've seen are from the old series, or are the children of the old gang. It really helps you to appreciate them more, knowing where they come from and what their parents were like. : )
i agree with Traumends, the first series was great on its own and this new one looks just as promising. I watched the first series and loved every minute of it. but you don't need to watch the first series to understand this new one because of the big time difference
If you watch the arc with Aang and his group, you'll understand the references that they make more. I suggest this, but you can probably start with Korra's arc if you'd like.