I really like bleach but the biggest problem I see cited is that the soul society arc and the arrancar arc are the exact same arc, just with a few roles recast, which is honestly 100% fair, the only difference between the two is that we don't get any real connection to most of the arrancar that get fought, but also the arrancar arc is directly followed by the fucking deicide chapters so I feel like it makes up for it
Okay so, I actually hear this a lot and if it's how you feel that's entirely fine but I do believe the two arcs were intentionally made to mirror one another with a reason. Anyways, here's my personal theory on the two arcs.
The Soul Society Arc
In the Soul Society arc Ichigo witnesses first hand how Rukia is made to suffer through a system of tradition. Head Captain Yamamoto expects all of the Gotei 13 to follow their duty and Byakuya, as the main antagonist against Ichigo is a living symbol for the traditions and rules being held above all in the Soul Society.
Ichigo and his allies witnesses firsthand the flaws in this system as they fight off against he various members of the Gotei 13. Rukia is ment to be executed for breaking the law in what headcaptain Yamamoto calls justice but meanwhile the Gotei 13 has people like Kenpachi and Mayuri who are literal criminals and commit acts of cruelties and they're heralded as the great captains of their divisions.
The soul Society is build on rules and traditions but in following it blindly it lacks justice. This is also why Ichigo never considered Byakuya his enemy. Ichigo isn't trying to fight Byakuya, he's fighting the sytem of the soul society as the Arc tests Ichigo's personal code against the stern rules of the Soul Society.
In Head captain Yamamoto's Words
"no personal justice takes precedence over the justice of the world" And ichigo's journey through the soul society is just that. A personal Justice fighting against a set of rules ment to maintain the world's justice.
So in the Arancar arc many themes repeat, the group racing into an unknown new world to rescue Orihime but there's immediatley a couple things different. As Aizen clearly states, the Arrancar aren't his allies, they are all loosely tied by the same goal but they're wild, unpredictable, several shinigami even refer to them as akin to animals. Kubo also states that the Arrancar have a spanish theme to signify their freedom.
Hueco Mundo is the mirror opposite of the Soul Society. It's an entirely lawless place. Aizen tells his subordinates not to trust him but on pure instinct alone they do anyways as Instinct is the only thing the denizens of Hueco Mundo know.
Instinct is the core principle for the Arrancar Arc as up to this point ichigo has been able to use his head to solve all his problems but as he's told, against an enemy like the Arrancar, this won't work. Ichigo has to learn to stop using his head to fight and instead trust his instincts.
His instincts are the first to tell him whats wrong with Orihime's dissapearance, his instincs teach him how to fight opponents that can't be reasoned with and as all else fails, his instincts protect those he loves. (even if another one of his allies is caught in the crossfire afterwards.)
The Soul Society and Arrancar arcs are similar because they're mirrors to each other and each teach ichigo valuable lessons he takes with him for the rest of the series.
TL;DR
The Soul Society is Ichigo fighting a place of law and order through his personal code where as in the Arrancar arc, Ichigo enters a place of lawlessness that he can only survive by using his instincts.
One teaches him how to conduct himself as a warrior, the other teaches him how to fight like one.
