Considering how much fun Avatar was to watch, its ending was really disappointing.
The events and fights were an incredibly underwhelming conclusion to what was a 3-year journey. We see random little bits of the liberation of Ba Sing Se but no major obstacle being overcome, Sokka and Toph (two of the biggest characters in the series) end up being in a small operation to take down about 10 warships instead of commanding an army or having a respective major fight of their own and after all the build-up to it, the fight between Ozai and Aang is just bland and unfulfilling. Credit where it's due, Azula vs Zuko was a good, satisfying fight, capping off a very solid character arc for Zuko, plus giving Katara a major role to play. Even Azula's rapid descent into madness worked quite well (even if there was no other real point to Mai and Ty Lee switching sides) but other than that, the ending was pretty poor. This is compounded by the fact that the siege on the Day of Black Sun had a much better set-up as a finale but that's another conversation.
First and most importantly, the fight between Aang and Ozai. For a start, the action was mostly shown in wide shots and we didn't really get to see much of either character's fighting style. Combat sequences are one of the show's main strengths from the beginning, so this was doubly disappointing. Next, there wasn't really a point at which both fighters went all out at each other, as equals. First Ozai was far superior, then Aang suddenly entered the Avatar state, becoming the far superior one, and then it was over. The inferior opponent in both phases spent the whole time running away, so there really wasn't much actual fighting at all. The fight wasn't just unsatisfying but it also didn't conclude any of the plotlines relating to it very well. Firstly, Aang's big advantage was the use of the Avatar State, which he made a conscious decision to be unable to use in season 2, so why was he suddenly able to access it? Next, Aang's solution to stopping Ozai was to bend his energy. This is something that is never mentioned nor even hinted at throughout the previous 60 episodes and the knowledge of how to do it is given to him literally the episode before, by a new character who apparently isn't part of the show's 2 only known worlds. The solution should have been something that was a running theme throughout the show, and more importantly something that was presented on screen (e.g. Roku's flashback which showed that anyone could be good or evil). Essentially this also meant that the solution wasn't mastering all 4 elements, the entire premise of the show, but instead the Avatar's innate power, meaning the past 3 years of training were basically pointless. Further to this, from a plot perspective, Aang hadn't even mastered all 4 elements before the final fight! Why hadn't he when the writers had 20 episodes for each one? Again, that's the series' entire concept and they essentially just binned it.
Pointless journeys are pretty much the theme throughout the ending. Again, Zuko's story is the one that stands out as well-resolved with a clear beginning, middle and (very satisfying) end, with backstory and motivations for it all. Full credit should go to the writers for that but every other character's arc basically gets trashed.
What was the point of Toph's story? She left home to experience freedom and adventure, she went on this incredible journey and then what? What was the outcome? Did she return home with a new resolve or outlook?
Sokka was supposed to be a strong warrior and brave leader and his feelings of failure on the Day of Black Sun were a perfect set-up for him to put things right in the final battle. His involvement however wasn't in a leadership capacity at all. As mentioned, he instead takes out about 10 war ships, with a small operation, nowhere near any of the major action. Did he learn anything from this? Did he achieve his goals?
Even Katara, who set out simply to become more than a kid from a small village, got no resolution. She became a hell of a water bender, but for what purpose? Does she now usher in the next generation of water benders in the Southern Water Tribe? Does she decide she needs to be a person of the world and keep exploring it?
All of these characters' journeys were great throughout the series, they just really weren't concluded at all by the finale. The final scene of the gang in Ba Sing Se just doesn't cut it. Half of these plotlines could have been capped off quite nicely with a simple ending scene for the respective character, showing what their 'happily ever after' looked like (viewers shouldn't have to read comics or watch other series for that closure, again this should all be on screen). For some absurd reason, yet again it's only Zuko who gets this solo ending scene and clearly later gets to go and answer more of his own unresolved questions.
Beyond this, there were also plenty of random story elements that were introduced in season 3, that went absolutely nowhere. These stand out because addressing the main storylines would have made the ending good but addressing these smaller points would have made it amazing. That said, these are probably more of a criticism of season 3 and the show overall than just the finale.
For me, these are: Toph having romantic feelings for Sokka, Katara learning to blood bend, Aang consulting 4 (FOUR!) past Avatars for advice that helped in no way whatsoever, Sokka's new sword that he literally hurls away and never sees again, Combustion Man (who was perfectly set up as Zuko's way of proving himself trustworthy but then kills himself thanks to Sokka) and the kids (Haru, Teo and the Duke) escaping the raid only to crawl through a hole and never be seen again.
The cherry on top of it all, the shoe-horned and poorly-executed romance of Aang and Katara finally comes to its apex (and also made for a weak af final shot in my opinion). Aang liking Katara but her not liking him back would have made much more sense given everything we'd seen throughout from both characters. It also would have made a much better ending, not just because of the incredibly tired trope of the hero ending up with the main female character but because it would have also completed Aang's journey. Not only is he the Avatar (responsible for the whole world) but also a monk. Letting go of his feelings for Katara would finally have aligned both of those elements of his character and allowed him to live freely and unnatached from any one person (not to mention explain his miraculous use of the Avatar State). But this is a kids' show and I'm a 25-year-old who just wrote about 1,000 words about it.
As I said, the ending is disappointing but I still had fun watching the show. I'm now really intrigued to watch the Legend of Korra. Hopefully they worked out some of these faults and hopefully I can get a bit more closure for some of these characters.