1. The Hogyoku's ability was to realize one's wishes - This just ruined the entire plot device for me as it cheapened the accomplishments of Ichigo and his companions and, to this day, just sounds really fucking stupid. IIRC, Aizen's explanation doesn't even make sense in regards to Vizards since the Hogyoku didn't work so it required an entirely different solution. There's no reason the Hogyoku couldn't have stayed as a tool for breaking down the barriers between species which would
still make Aizen freakishly powerful by fusing with it.
2. Aizen masterminded Ichigo's battles and development - Again, cheapening Ichigo's accomplishments while being just generally implausible in how he could pull that shit off. It was a stretch in the Soul Society Arc and this was far worse.
3. Ikkaku has a bankai - I freaking loved this fight as a kid. Especially as that was back when I was an anime-only watcher which did an awesome job with it. But in time I realised that Ikkaku never should have had a bankai in the first place as it was totally wasted on a loser like him. Now I just shake my head.
4. Segunda Etapa - A second release doesn't even make sense and it was a completely pointless upgrade considering Ulquiorra
already outlcassed Ichigo by a wide margin so there was no noticeable difference between the releases from our perspective. Kubo should have just used that appearance for Ulquiorra's release and just left it at that.
5. Yammy was the 0th Espada - Zero, in this case, meaning a total loser. It wouldn't have been so bad if he'd actually been a dangerous threat that nearly defeated all of the captains (or maybe required Unohana herself to kill him) but he was just used to waste time and panels. He should have been left as the 10th Espada and gotten killed by Chad as vengeance for his arm.
6. Final Getsuga Tensho - The definition of an Ass Pull. Or would that be Deus ex Machina?
7. Chad is a Fullbringer - What a goddamn downgrade. To this day, I continue to believe that this is just a misunderstanding on Chad's part as he tried to pigeonhole his power to fit what Xcution described. He awakened his power entirely because of Ichigo and will one day have an entire set of armour, I swear.
8. Ichigo had Quincy powers - Human/Shinigami/Hollow/Fullbring/Quincy hybrid. Need I say more?
9. When a bankai breaks, it cannot be fixed - Because it's not like Renji's bankai got reduced to woodchips, including even lost a part of its skull, and looked perfectly normal the next time? Everyone bitching at Ikkaku about not using his bankai against whaleface falls pretty flat. That was some contrived bullshit. It'd been fine if we'd been told this from the very beginning and avoided seeing all these bankai gets wrecked but noooo...
10. Zanpakuto were created by one of the Zero Squad - This totallly ruined the mystique of zanpakuto for me. Shinigami shouldn't need this guy's tools just to wield their own inner power.
11. Sode no Shirayuki's lowers own body temperature - This would have been pretty cool (heh) if Rukia had had this ability from the beginning because it'd seperate her from the blatantly superior Hitsugaya (who even goes on to overshadow her bankai with his final form's strongest attack) rather than the being a lesser copy. But introducing it like this didn't work for me and the whole "I'm so cold I'm actually dead but controlling my reishi to move" or whatever was some weird shit.
12. Hihio Zabimaru is actually So-oh Zabimaru - Again, this should have at least been alluded to from the beginning. Zabimaru should have told Renji from the beginning that he wasn't good enough for bankai so he'd only be able to tap into a portion of his power, then he should have learned the true name directly from Zabimaru when he was ready. Not when some bald guy with nonsensical powers just blurts it out.
13. Hichigo is Zangetsu, Old Man Zangetsu is Yhwach - I loved the idea that Ichigo's zanpakuto spirit is a Hollow and as great as old scenes like
this were, replacing Old Man Zangetsu with Ichigo's Hollow would have been awesome. Aside from that, I hate everything about this twist.
14. Nanao inherited a zanpakuto - I actually liked the basic concept of a special zanpakuto that has been passed down through generation, as well as the development of Shunsui/Nanao's past and relationship. But they should have been two separate things. In fact, Yoruichi should have had the Shinken Hakkyouken. It would have perfectly explaining why she never used a zanpakuto and fits her already known background as the leader of a noble clan.
15. Yachiru was Kenpachi's zanpakuto - It could have worked. It really could have. But giving her her own zanpakuto with a shikai and having Kenpachi discovering his zanpakuto's name without her being around just made it bullshit.
16. The Soul King - Fuck everythng about this guy.
Everything.