They even get mecha robots, and inventions, unlike in Star Trek, stay invented and impact the setting and plot. That always irritated me about Star Trek. A useful invention in one episode is forgotten in the next one. In Star Trek they're always recalibrating the big deflector parabolic dish into a weapon of some kind. Why not leave it that way? Meh. This is why Star Trek is BAD scifi. So is Star Wars, btw. Most of the plots of star wars are about planet destroying space guns. And moichindizing.
Korra isn't like that. Korra is a world with some quirks, with a small percentage of the population having super powers that are useful for building and destruction, and even industrial purposes. They've got scary portals that rip through to different dimensions with spirits inside, and the spirits come out, and live around the people, with their own peculiar motivations. The series is about Korra gaining awareness of the parts of life she wasn't born with. Its a proper coming of age story, and by the final episode she understands things enough to know what's right, what is worth fighting for, and the cost of losing.
Xander is the friendly version of Iroh, the only wise character in the original Avatar, and one worth watching because his approach to the civil war, to avoid the stupidity, was a great counter to Zuko, so passionate at attempting to regain his honor against a king so evil he burned his own son's face. Zuko has a great character arc, completely unlike Draco Malfoy, who was merely an opportunistic racist parasite. Rowling wasn't very skilled at writing villains. Avatar is better in every way, and so much more satisfying by its conclusion.
I wondered if having Xander around Korra would allow him to activate his superpower. In the fanfic, Xander becomes the obsession of Zuko's insane lightning shooting sister. In Korra, would he have stopped the mecha being built by distracting the Metal Empress from finishing her purple-maser weapon? Probably. Xander's superpower is being irresistable to monster-women. Kuvira is relentlessly ambitious as well as competent. Republic City wouldn't have gotten so damaged if Xander were giving her a creepy smile and a funny limp. Would Xander have been able to offer better advice to the love triangle of Korra, Asami, and Mako? Probably.
As for the ending, I totally understand the implied result of the series, of Korra and Asami becoming a couple, presumably lesbian. However, I am uncertain if that's the sort of thing that will last. At their young ages, early to mid 20s, it probably is convenient, but when the burn for babies comes, they'd drift far enough for husbands and families of their own. Even Toph's situation (ending up a single mom whose kids resent her) proves that the implications of the adventure (Sokka is NOT the father of her kids) don't last when things calm down again.
And finally, Korra ends the use of the setting because too much technology was invented to allow the charm of the contrasts between magic-bending and manmade tool-technology to work anymore. The first Avatar was mostly peasants, with the Fire Nation having metal steamships used for war, but little trade, everybody else limited to peasant living and the spaces between towns largely wild and untamed. The common high-tech was the wheel on a cart, and cities had stone monorails. That isn't so magnificent, and all required lots of benders to keep things going.
By Korra they'd figured out tools to manufacture things, and knew enough to have lots of airships and steam trains and good steel. They had cars and stoplights. Things were 1920's civilized. Anything more advanced than that and you'd lose the charm. So Korra ends the Avatar series, and it was a satisfying ending.