I will separate this in 2 main points, Naruto and Sasuke. Of course, there are a lot more things in this arc but with both Naruto's and Sasuke's character development is enough to place this arc as one of the best in Shonen. Naruto -It showed how Naruto is not perfect, that even though he is strong, he's still insecure and suffers from anxiety(which is basically a deconstruction of the typical shonen mc). Kishimoto isn't afraid of letting Naruto fail, he has a lot of failures since he brought Tsunade back, he kind of broke the streak with Kakuzu's fight, but after that, he fails again in retrieving Sasuke and I wouldn't consider his fight against Pain a success , given that he almost breaks the seal and destroys everything(including Konoha) and killing himself in the process. -This is the arc Naruto is faced with the cruel reality of the Shinobi world. He made a lot of promises and all of a sudden all of them seem to contradict each other and it looks like the world itself is against him. He wants to save Sasuke and protect the village yet Sasuke seeks the destruction of Konoha, and Konoha wants to kill Sasuke. Protecting him could lead to a war with Kumo, fighting and killing him would protect peace but break his promise and probably Naruto as well, it's obvious that he doesn't want to kill his best friend, after all. He struggles to find an answer to peace and on top of that now his friends are ok with killing Sasuke...Not to mention that he's still a teenager, a lot of people don't like the panic attack scene but I think that it's more than justified, it's realistic and a nice touch to show Naruto's anxiety and stress. Sasuke First of all, I know that a lot of people dislike Sasuke because they don't get his character at all so I will put you on context. It’s pretty obvious that he’s not a sane person(understandable because of all the shit he went through). He wasn’t sane even before Itachi’s reveal so it was obvious that he would end up even more insane after that. So now we have a traumatized teenager with no guidance at all -something very important, at Sasuke's age and more importantly in his case, it'd be essential to have some sort of guidance yet he has none- who acts like everyone would act in his situation, taking the worst possible decisions. -This arc shows us Sasuke's descent into darkness, how far he is willing to go to in order to fulfill his goal, his insanity, his twisted sense of pride and his incredible black and white view of the world. To Sasuke there is always an "us vs them" situation, he labels someone as the enemy and then is incapable of thinking that maybe the other guy is not that bad, that it could be more of a gray situation. Which is also helped by the fact that he is broken and tries to repair himself by seeking revenge. -Less cool: The anti-hero trope, in any media, they often use the traumatic childhood of the characters, as an excuse to make them cool and badass, which couldn't be more stupid. You have guys like Lelouch, Accelerator, etc. That due to their tragic and traumatic childhood, become cool and badass. The thing is that that is a horrible representation of what traumatic experiences do to people, Sasuke's character is actually a deconstruction of this trope, he’s a realistic representation on how trauma can affect people, he’s not your typical character that is cool and badass due to his sad backstory. He starts as a cool badass in part 1 and the more Sasuke descents into Darkness the more unlikeable he gets and is less cool and badass and more insane and rotten. Sasuke is a fascinating and very well-written character and this arc shows us his complexity and insanity. -Sasuke also works as a mirror to Naruto. Sasuke is like a mirror to Naruto, he is Naruto if he was corrupted by the system, even Naruto says that he could have ended up just like him. Sasuke’s point of view is necessary because it shows us the “ugly side” of the Narutoverse, the story of a traumatized teenager obsessed with Revenge that is that way due to the fucked-up system.