I'll never forgive you Oda
Orochimaru never got justice. And I don't mean he wasn't taken to court, I mean Orochimaru was never given an opportunity to go all out in the series. It's criminal that Kishimoto decided to end things while having one of the first antagonists still display barely any jutsu throughout the show. After being revived and reconnecting with his hands, this is all Orochimaru displays.
Now it might seem a bit nitpicking to be upset Orochimaru never got to be super strong (like he should've been), but considering his goal is to master every ninjutsu and his philosophy encourages constant chaos to provoke evolution, I'd say this is a big issue. It's like the 8th Gate, it was hyped early on in the Chuunin Exams and we saw it in the final arc. Instead, we get Orochimaru standing on the sidelines. A man who invents ninjutsu, steals bodies and researches forbidden ninjutsu for fun.
This is the same character who's stats — an outdated relic of Kishi's own internal powerscaling but a useful tool nonetheless — are .5 below Itachi's. Clearly, he's an absolute monster. Furthermore, in the 4th Databook Orochimaru has every known elemental capability (mastered?).
I mean he even got a Zetsu body, made out of Hashirama's DNA and ideal for sage mode. The only thing Orochimaru lacked to become a sage was a suitable body!
Just, god. One big misstep for Orochimaru. He came back with the potential to be so powerful and got sidelined for zero reason.