Pretty sure all of the original is on Instant Watch. I haven't seen any of Korra yet-though I should.
Korra is experiencing flashbacks of Aang's life (reincarnation is a strong theme), so I think you need to brush up on a few episodes in order to appreciate the suspense. Plus, the first series is not that long. You'll see some differences between the show.
Oh thanks. I'm pretty sure sickVisionz can squeeze in five episodes a day (around 150 minutes/2.5 hours or so). He'll catch up in a week.
I'm the type to lose a lot of interest in checking out any completed series if I find out it's more than 13 episodes long. 50+ is basically a guaranteed not watch. Plus, I saw a few minutes of Avatar on Nick a long time ago and I've got no interest in checking the rest of it out.
@DCypher: Nooo, you reminded me that it exists! I was doing so well to forget about that dark day, but now I remember everything...
On topic its not necessary to watch Avatar: The Last Airbender (I personally got all 3 seasons and watched them before Korra first aired, took me around a week), but it does have plot points that you would not understand fully (At least at first) if you never watched the first series.
I just don't feel Korra the same way I did Aang, I think the way it's shot, the lighting, the atmosphere and the story are great. Totally on par. I just don't feel the same way after a few episodes of Korra that I did when I first watched Avatar. Just my feelings.