Tahno (in an extremely suggestive voice): Y'know, if you'd like to learn how a real pro bends... I could give you some private lessons.
Korra isn't too heavy on the subtlety herself:
Korra: Yeah, but when you're with her you're thinking about me, aren't you?
After Bolin catches Mako and Korra kissing, Mako finds him drunk at a bar. On noodles, not actual liquor, but the viewer can put two and two together here.
The next day he acts hung over, and "loses his noodles."
It's also worth noting that sake (Japanese rice wine) is sometimes served in a bowl.
"And the Winner is..."
Tenzin: Lin, with so much on the line, it would be nice if we could help each other out, at least for one night. (Emphasis added)
Lin: Like old times? (Now she's behaving all Tsundere)
Tenzin: Like old times. (Look at his face)
Lin: Ok, I'll try to be less abrasive than usual. (Emphasis is in the dialogue)
Oh, and speaking of Amon's Memetic Molester behavior... If that scene where Amon removes Tahno's bending doesn't count as that, then I don't know what does.
In general, all of Amon's scenes where he removes someone's bending either contribute to his Memetic Molester status or look like some kind of public execution.
"Skeletons in the Closet"
Tarrlok's and Noatak's backstory plays up much like a tale of child sexual abuse, with their abusive father bringing them along with him on "hunting trips" away from their mother and making them perform acts that Tarrlok sees as degrading and viscerally wrong. Noatak ends up chillingly stoic and seemingly depressed, being the father's favoured plaything, and ends up committing said acts with less resistence... until he violently retaliates against the father and tries to run away.
Book 2: Spirits
Ginger's demonstration of what the new invention of movies (or movers, as Varrick calls them) can do. We get the distinct impression that Verrick is looking to go into stag films.
In real life, when "moving pictures" were first getting popular, they were often used as ways for people to view "mature" material. The Rule of First Adopters, and all that.
Also a Shout Out to Aang's use of it when they first visit the Fire Nation in season three
Bolin shouting "My instrument!" after Mako heats up the hot tub water to the max.
Doubles as a Shout out to Toph's "delicate instrument" line in the original series.
Ping questioning Mako on dating Korra. His tone of voice and Mako's reluctance to speak on the matter make it sound a lot like he's asking him how the sex is.
Ping: Come on, if I was dating the Avatar I'd be telling everyone about it!
"A New Spiritual Age"
As one can see from the page picture, it's shocking on how they got away with showing a genuine human corpse in a more realistic stage of decay (likely naturally mummified by the dry desert air) instead of using the typical bare-boned skeleton. Good thing the shot itself was quick...
Watch carefully when Korra and Asami's car stops in the middle of the road during Korra's driving lesson. The frustrated driver behind them passes them, and very subtly moves as if to flip them off right before he moves out of the shot. For bonus points, he is also the recurring Man In The Yellow Feathered Hat background character.
Korra's talk to the new airbender in the bridge sounds suspiciously like she's talking him out of committing suicide.
The "airbender hipster" has rather suspicious stains on his clothes.
Ming-Hua and Ghazan are rather liberal with their mutual battle tactic of throwing their opponents into molten lava.
"The Earth Queen"
P'Li is said to have been in a relationship with Zaheer. When Zuko shows up to check on her, she figures Zaheer must have escaped and quips "I'm feeling warmer already."
Mako and Bolin's grandmother fearfully admonishing their cousin for speaking ill of the Earth Queen, then immediately turning to a picture of the Queen in her youth and paying homage is an extremely disquieting mirror of the fearful cult-of-personality that has been documented in North Korea.
When Korra asks Tenzin about how the Earth Queen's like, his answer seems like a Last-Second Word Swap from the "b" word:
Tenzin: I hear that she can be quite [beat, with a suspicious "a" sound] demanding.
"In Harm's Way
Ming-Hua apparently freezes a guard to the wall in a way that encases his entire head.
"The Metal Clan"
Lin and Suyin are half-sisters, which is unusual for a cartoon but not unheard of. Less common is the fact that neither met their fathers, which raises some interesting implications about Toph's love life.