Good idea for a conflict ruined by terrible pacing and even worse explanations. WTF is up with Jinora, Why did Korra decide to leave the portals open. Why could Korra become a Kaiju when logically everyone should've been able to. That sort of thing.
An exciting, but completely brainless finish.
Indulge me for a moment, though: I watched it at Nick.com, the official venue. I have good internet and it was constantly buffering, and I didn't even find out the episodes were up until a day later, so you know Nickelodeon had some serious traffic.
And I was watching NERF commercials? Nerf made sense for The Last Airbender, but Korra's following is significantly older (mostly old Avatar fans, who are basically older teens or adults now.) Advertizing Nerf basically wastes all of Nick's traffic because it's not an ad that many of their consumers will pay attention to.