For me, when I rewatched Book 1 I thought it held up a lot better than I remembered.
Pacing was pretty solid.
It was an introduction to the world and characters, but Korra/Tenzin/Lin had pretty good arcs throughout.
Mako/Bolin as characters held up better in the first half. Felt like the writers didn't know what to do with both of them after the Pro-Bending tournament. Mako then was characterized by his role in shipping and Bolin became the purely comic relief character, whether it's appropriate or not which stayed true to his character throughout the series.
Asami seemed "perfect" to a certain degree, but her facing her father this arc is probably the best arc her character ever went through. Her fight with Hiroshi I think was the most emotional moment for me, mostly since I got an "epic" vibe from Book 3 more than felt connected to the cast/goings on with the villains.
Both villains were probably my favorite villains of the Korra series and both had pretty interesting and somewhat complex (though others would argue convoluted) stories, and both Tarrlok and Amon had pretty solid plans throughout.
And pro-bending might be bad for some since it ultimately did nothing for the plot overall, but during the rewatch I liked it for the most part minue one or two hiccups. Even the "baddie" of the tournament was kinda fun in a hammy sort of way, though I'm bothered that they basically cheated the entire match despite being known as the best of the best.
The bad parts were the focus on the shipping, consuming characters and plot throughout the series, and various deuxs in the ending that had to happen due to poor planning on Nick's end like Wagomu said. Korra getting back her abilities, the way she learned airbending… and I'm one of those okay with Amon's downfall, and even Korra's plan to take him down since she and Mako weren't exactly bright bulbs at that time. Also, Iroh III for me was fanservice that rubbed me the wrong way.
Still wanna try to rewatch season 2 marathon style again to see how it holds up. It's not like I don't exactly have the time right now...