Something is funky about this. Not even counting the voices (which easily could've been messed with to get around youtube filters that would take it down) it just doesn't have the feel of Korra to me. It seems unpolished, like it isn't actually the final cut.
The second clip gave me that sensation too. I'm almost sure that that clip is not the finished version we'll be seeing in Nick this September. Although i have to say, that even it looks unfinished, i'm happy to see the great quality so far, looks like an Anime!