I can see people being upset with Aang not having to kill Ozai, but that was really not poorly done at all.
The last half of season 3 dealt with Aang struggling with how to deal with the Firelord. He has to be stopped because he is clearly a monster and incredibly powerful, so if you don't deal with him permanently he will still cause trouble. But Aang isn't sure he's capable of killing the guy. Yes, energy bending did come a bit out of nowhere, but it fit perfectly in line with the development Aang was having and was a way of solving the situation his way. Energy bending itself wasn't foreshadowed much, but Aang feelings were and that's what really mattered.
I don't even think they tiptoed around the idea of killing just because it's a kids show. Sokka was quite clearly shown to be willing to kill the firelord. Aang had his own ideas and that was fine.
If you were to compare that to the equivalent part of Korra, it's the final Aang show up where he cures Korra's bending problem. Similar to energy bending, that kind of appeared out of nowhere. Except the reason that happened was because... Korra hit rock bottom supposedly. Literally right there. There was nothing leading to this scene to expect this other than "lol everything will work out in the end" Korra never pondered "what will things be like if I lose my bending?" She was just like IMA RUN IN THERE EVEN THOUGH WE ARE VASTLY OUTNUMBERED AND BEAT UP AMON. Now that she's lost her bending she's just suddenly suicidal or something. It's not exactly an unreasonable reaction, but with the amount of time devoted to it, it's so poorly done.
While I'm at it, I think a lot of people have the wrong idea about Ozai. They think he's "bad" or "poorly done" because he doesn't have the depth other characters like Azula, Iroh, or Zuko had. He's mostly just an incredibly powerful evil bad guy archetype that wanted world domination. There was really nothing wrong with that. The series primarily focused on the journey of the Gaang and the people they encountered as they built up and prepared for that final encounter. The characters that they encountered were all very colourful with all kinds of things going on. Ozai had one purpose: to be the power hungry tyrant Aang had to defeat. He had that all in spades. He had the arrogance, he had the endless amounts of power, and he had the ruthlessness. Just looking at him in any scene he was in and you get that vibe. That was really all he needed to be and he did great at it.
Now, that means he's not going to be ahead of any major characters in rankings, but that doesn't really matter. It was great for the show for him just to be what he was. They could have added more facets to his character, and maybe if they had more episodes in the final season they would have, but all the show needed out of him was what it got, and there's really not a problem with that.
I could see being a bit disappointed with him just because you might want him to be as fleshed out as the rest of the cast of the ATLA. ..... But clearly you guys aren't applying that metric to anything in Korra.
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The batman villians all seem to be one big joke that batman refuses to laugh at - SantaRPG