The Legend of Korra is not "dark"? Lol.
The creators had trouble with releasing it on Nick, because it was too "dark", respectively, too mature for the common audience of Nick. "Just an american cartoon", that shows that you have no idea what the shows of the Avatar-franchise are about.
Usually I don't use such terms, but honestly, The Last Airbender and Legend of Korra both shitstomp crappy series like Naruto, Fairy Tail or Shingeki no Kyojin without effort. The characters are not only more likeable, but also more developed and the whole world built by Michael DiMartino and Bryan Konietzko and their teams is so fascinating and interesting.
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Legend of Korra's plot and character development alone trumps Naruto in every way. You also don't find asspulls, countless powerups and plotholes there. Usually those people who diss the two shows, are the ones who only like plain, flashy anime with lots of explosions that have one pseudo-cool, male main character who defeats everyone.