• King Bumi, a man who decided that it was better to surrender his quite frankly impenetrable fortress of Omashu to the enemy (despite having every single advantage imaginable) instead of fighting because "something neutral jin energy", thus losing the second most important Earth Kingdom city and causing a refugee crises that burdens the capital city of Ba Sing Se.
• Earth Kingdom soldiers who can control and create secret tunnels wondering how to get out of occupied Omashu..... Despite the fact that as Earth Benders in a city made out of rock they can literally just leave whenever they want.
• Long Feng not using his brain washing technique to simply brain wash the Earth King like he does everyone else.
• Long Feng simply not using Earth Bending to bury the offending giant siege drill that he needed to hide from the Earth King.
• Earth Benders choosing to launch ineffective small boulders to stop said aforementioned siege drill instead of just burying it into the ground with their Earth Bending where it would be helpless.
• Adults giving important war documents to children of other nations to deliver to other adults instead of using a dedicated messenger or just doing it themselves.
• Sokka's dad spontaneously deciding to get off of Appa in order to attack a turret tower so the plot can have him be injured so that Sokka can shine.
• Iroh saying he cannot fight his brother Ozai for the Phoenix King throne because "a sibling power struggle would look bad" only to immediately send Zuko to go deal with his sister, the recently crowned Fire Lord Azula.
• Iroh's entire idiotic endgame plan of sending children to do the most dangerous and important tasks of stopping Azula and Ozai while all the available adults unnecessarily attack the occupying Fire Nation force at Ba Sing Se while it is at it's strongest.
• The Water Tribals not figuring out throughout their entire existence that the two completely ageless fish that ceaselessly swim in a circle in a tiny pond in their sacred grounds are in fact important.
•The Water Tribals doing absolutely nothing against the Fire Nation force attacking their capital. They control water but don't use the sea against the invading Fire Nation soldiers. They wait until said soldiers are literally in their city before doing anything.
• Zhao getting promoted by Ozai from Commander all the way to Admiral after doing nothing but consistently failing. That is one colossal leap in rank for a boatload of failure.
• Iroh's plan to move his tea business to a fancier district of Ba Sing Se where he is likely to bump into Earth Kingdom generals that fought him thus blowing his cover.
• Sokka and Toph just leaving an important Earth Kingdom general to his fate while he is being captured by the rebellious Dai Li.
• Earth Kingdom generals and the Earth King using the Dai Li, who literally just got ousted as being disloyal to the Earth King, to arrest their leader Long Feng. Screw using 100% loyal Earth Kingdom soldiers I guess.
• Earth Kingdom generals having zero guards around themselves after both a Fire Nation attack and a revelation of the Dai Li's treachery.
• Sokka has a list of folk for his dad to recruit which includes two individuals (Boulder and Hippo) who last he met were going to sell Aang to the Fire Nation and had zero remorse over it. Really Sokka?
• A large group of Fire Nation nobility party goers somehow don't recognise Prince Zuko and Princess Azula right after a major speech was given that had both of them present. This was after it was shown that even Earth Kingdom peasants in the back of nowhere know of Zuko.
• Katara never teaching Aang how to use water healing. Seems like a pretty obvious choice to have an extra healer in the team and the pacifistic Aang should especially love an ability that helps others.
• Sozin going to Avatar Roku's aid, saving him, needing to be saved by him and then suddenly betraying him which was apparently his intention all along.... Even though he could have simply not gone to Roku's aid at all which would have killed Roku anyway and not put Sozin himself in any danger.
• Characters also switch from one extreme to the other in back-to-back episodes. Aang can go from nearly killing himself trying to get to Ozai by himself to suddenly goofing off by attending Fire Nation schools. Most jarring was when an ally of the Aang Gang died and the beginning of the next episode has Sokka saying "We are finally getting some good luck".
Yikes.
• Occasionally, abilities also come up that are rarely ever used again. Prime examples that come to mind are Sokka's club's vibration enhancing ability, Toph overcoming her weakness to sand, the Earth Kingdom's freaking homing bombs that only get used for fireworks of all things and Katara's Blood Bending (a subset of Water Bending) which never gets meaningfully utilised after a single episode.
Yeah, I could keep going at this for a while. Time to move on.