I actually have been thinking about this and that was the trade off between ATLA and LOK. ATLA spent a lot on relationships and character growth while somewhat shoving the spiritual and political aspects to the side because they were in a war. Yet the romances that blossomed between characters still felt natural and healthy, even when they weren't put into the spotlight.
*But in LOK while there is a big bad every season, they focused more I believe on the spiritual and political aspects of the Avatar world and made relationships second. Do many of the secondary characters have healthy, good romances? Yes many do such as Tenzin and his wife, Opal and Bolin, Jinora and Kai, and even Zhu Li and Varrick's relationship we knew was going to end with them getting together. Their romances felt natural when the spotlight was on them and off, with their interactions seemingly genuine and true. And from what we've seen of Lin, we know the right guy or gal is just around the corner for her.
*But when it comes to the romances of the main characters- Makko, Korra, Assami, even Zaheer and his lady love, they don't feel healthy and actually feel forced. My prime example of this is Zaheer and his lady love from season 3. Their relationship felt like Korra and Mako's in Season 1-forced. The chemistry between them feels artificial and like one of them will backstab the other in the back when they get the chance. So when his love dies and Zaheer loses his last earthly tether, it hit me like "oh, he really loved her" which wasn't the reaction I was supposed to get. The reaction I was supposed to get as "BUT HE LOVES HER SO MUCH! THEY WERE MEANT TO BE TOGETHER! HOW COULD YOU-". But because their relationship hadn't been fleshed out enough by the writers and in the story, I never got that reaction. With them, they could have done a flashback sequence to how they meet and fell in love, doing little dates on the side of kidnapping Korra, just something to cue us in to their romance.
*Which then leads me to the very reason for this thread:korrasami. The reason it came as so much as a shock and surprise to many isn't because there were never any clues or because we were "looking through a heterosexual lens" like one of the writers said in an interview. It's because Korra's only romance before Assami was with Makko and it felt SO forced. It felt like she only ended up with Makko because that's what the leading lady does-falls in love with the leading man. They never had much in common in the first place and the secretivity on Makko's part was bound to destroy their relationship sooner or later. And when Korra hooked up with Assami, it again felt forced, like more of a statement then something natural. Could after years of being friends they realize they have feelings for each other and gotten into a relationship? Yes they could have.
*But can you really convince an audience that after one failed relationship on both parts, no dating other people, and only corresponding for three years in 4 letters to each other that Korra and Assami were going into a relationship? Not really. Could the blame also be put on the shortness of the series and the mess Nickelodeon did with handling the series and the stress it put on the creators and staff? Definitely. However, take into consideration that ATLA was able to give Zuko a complete and healthy romance in as little as half a season with his lady love.