"Book: Air" was a very common name for fan sequels, in keeping with the Theme Naming of the show's seasons, as it was the only main element remaining that did not have its own season. The first season of Korra is titled "Book One: Air".
When the series was initially announced as The Last Airbender: The Legend Of Korra, some fans rejected the Artifact Title and its association with the much-disliked film adaptation, instead referring to the series as The Legend Of Korra. Official sources soon followed suit.
Two of Aang and Katara's kids are named Kya and Bumi, after Katara's mother and Aang's oldest friend. Fanfiction writers named hypothetical daughters of Katara "Kya" years before The Legend of Korra was ever announced.
Jinora Gran-Gran, I've been reading all about your old adventures; I've been dying to ask you; what happened to Zuko's mom? Katara: Well, Jinora, it's an incredible tale— Ikki:[interrupting] Gran Gran, you look old. How old are you? Why is it so cold in the South Pole? Can we sit around a fire and play games and tell scary stories and make snowmen? And then could you make the snowman move with waterbending and chase us? Wouldn't that be fuuuun? [Jinora hangs her head in disappointment]
Brolin for Bolin. Even more appropriate now that we know about his lady-killing ways.
Mama!Mako is becoming quite popular on account of Mako's doting and protective ways.
Sparkly Sparkly Bush Man for the park-dwelling vagabond Korra meets in the first episode (after Sokka's temporary name for Combustion Man of the original series). Secondary sources later revealed his real name as Gommu.
Korralition or Krew for the new iteration of Team Avatar, as their in-universe team name, the Fire Ferrets, wasn't known to the fandom until early March 2012.
Spider-Lin for Lin Beifong after her actions in Episode 6.
Many of the characters have earned themselves superhero themed nicknames. Lin has also been dubbed "Iron Lin" for the way she puts on her armor, and "Wonder Lin" both for being amazing and the fan theory that Toph created her via earthbending. General Iroh has earned the nickname "Iroh Man" after the finale, and Mako has been dubbed "Batmako" for his similarities to Batman in episode 3.
Big Daddies for the mecha tanks introduced in episode 7, especially due to the diving suit inspiration.
Korramobile for the Satomobile the Krew rides in when hunting Equalists in Episode 8 is somewhat popular given all the Batman undertones.
"Honora" for Zuko's daughter (the current firelord) who has yet to appear in the show.
Friday Night Death Slot: Book 2 aired Friday night instead of Saturday morning. By the numbers, it is down about 1-2 million viewers per episode than last season. And then they push it back another hour without warning, leading to another drop of a million viewers.
Genius Bonus: Hiroshi Sato was based onHenry Ford, head of the Ford Automobile Manufacturing Company. Another fun fact about Ford was that he was rumored to have supported the Nazis. Take this into account with Hiroshi's character.
God Never Said That: Despite the quote from Andrea Romano being that the original cast might as well be dead, for all of the impact they'll have on the plot, numerous people still insist she declared the entire cast to be dead and gone, accusing her of being mistaken or lying when it was proved otherwise.
Episode 6: Lin was the third person in Tenzin and Pema's Love Triangle.
Episode 7: One of the Satos is working with the equalists. It's been a long time theory at least one of them is. Also, mechas. Super bonus points for those who saw the blue prints way back in episode 1. Also in-universe with Korra having eavesdropped.
Episode 9: Tarrlok is Yakone's son, and Amon has a means of protecting himself from bloodbending.
Thus far in the finale: Tarrlok is Amon's brother—his younger brother. Tumblr went on fire for that one.
And Amon is a bender—and a bloodbender.
Pretty much every major plot point was successfully predicted. It's only a matter of time before a waterbending Serial Killer named A. Firebender shows up.
Combined with Hilarious in Hindsight is that Avatar/Naruto crossover fics were common due do the fact that the designs weren't horribly dissimilar. Cue Studio Pierrot, Naruto's animation studio, animating for Book 2 of Korra.
"The Sting". Many people were not the least bit surprised that Varrick turned out to be a Corrupt Corporate Executive who staged the attacks on his own vessels so he could buy out Future Industries.
"Harmonic Convergence". Unalaq reveals his master plan, to become the Dark Avatar by merging with Vaatu like Wan did with Raava. Lots of people saw that coming a mile off, but most dismissed it because it was so ridiculous. Joke's on them, eh?
Name's the Same: There's a real person named Hiroshi Sato; a retired Japanese football player.
No Celebrities Were Harmed: While not closely visually related to their historical equivalents, some characters share features of famous people of the World War era.
Varrick's eccentric mannerisms, quick temper, and business layout are very similar to that of American media mogul William Randolph Hearst. In the episode "Peacemakers", he furthers the similarities by openly filming the civil war and splicing it with fake footage to make it propaganda, a practice Hearst used in the Mexican theatre of WWI.
Playing Against Type: Steve Blum as Amon, Clancy Brown as Yakone, and Lance Henriksen as The Lieutenant are the only major exceptions so far. PJ Byrne starred in quite a few medical and/or legal dramas before being cast as Bolin. Mako's actor David Faustino was best known for his role on Married... with Children. Mindy Sterling (Lin Beifong) and Janet Varney (Korra) are best known for comedic roles. And while Dee Bradley Baker isn't new to voicing menacing villains such as Tarrlok, it's certainly not something he's necessarily well known for. There's also JK Simmons, best known for playing hilariously foul-mouthed, choleric Jerkasses, as Tenzin, the most calm and spiritual member of the cast (though he has his moments).
Production Throwback: Naga the Polar Bear-Dog, Korra's animal guide and occasional transport for the new Team Avatar, was originally going to be Aang's waaaay back before the series was fully realized- sketches and concepts for this can be seen in the Avatar: The Last Airbender official art-of book.
Promoted Fangirl: Janet Varney was a big fan of the original series even before auditioning for the role of Korra, needless to say she was stoked to hear that she'd booked the part.
Screwed by the Network: Book 1 was aired on Saturdays with a great deal of promotion, and it got very high ratings. But for Book 2 the channel gave less promotion and moved it to the Friday Night Death Slot. The Book 2 premiere only had half the ratings of the Book 1 premiere. Then with little warning, the show was pushed back an hour and ratings fell in half again. See how bad the numbers got. Whether intentional or not, the network screwed this show hard.
When Bryan Konietzko announced the time change, he added that it may have been done to try and counter-act this. It kinda worked for episode 9.
Throw It In: A scene in "The Spirit of Competition" where Mako is angrily walking through a door and completely ignores a fan trying to say hi ("Hey Makooooo-oh") originated when someone drew a random extra standing next to Mako in the animatic and the episodes director thought he looked disappointed.
There was an In-Universe case in a mover where Bolin kisses an actress in a scene where he wasn't supposed to and the director decided to keep the kiss.
What Could Have Been: Asami Sato was originally intended to be a villain, but during the first season's scripting process Mike and Bryan decided that 1) she wouldn't have much use in the story after her intended mid-season villainous reveal, and 2) was too cool a character to waste that way.
Working Title: First Avatar: Legend of Korra, then The Last Airbender: Legend of Korra. The "The Last Airbender" Artifact Title was ultimately dropped, while the "Avatar" supertitle was retained for overseas markets.