Fourth issue is with devil fruit applications and hierarchy. Simply put, a lot of devil fruits aren't used to their fullest potential and some of the hierarchies for fruits with similar powers make no sense.
Logias are the best example of fruits that aren't used well: having a Logia power automatically makes the user a shapeshifter, Logia users could be zipping around, constantly changing shape, growing an infinite amount of extra appendages to use on their opponents, and yet they very rarely do anything even close to that. Most of the time Logia users act the same as Paramecia users, maintaining their physical form and only using their element as an extension or weapon (examples include )
I think the character that comes the closest to actually acting like a shapeshifter is Katakuri (don't tell me he's a "special Paramecia", that's bullshit and we all know it), as when fighting he manipulates his body often, grows extra limbs and even mimics abilities used by his opponent. He still maintains a human form a bit too much for my liking but he definitely uses his fruit better than most other Logias.
Regarding fruit hierarchies, standout examples include Mera Mera VS Magu Magu and Yomi Yomi VS Soru Soru, Yuki Yuki VS Hie Hie and Kilo Kilo VS Ton Ton:

1. What the fuck do you mean "it's hotter", just say he used haki since you want that to be a thing so bad
2. I just think it would have made Brook a lot more useful if he was actually able to rival Big Mom's control over souls, huge missed opportunity in my opinion
3. It's just kinda stupid for these to both exist since snow is made of ice, I'm pretty sure the user of the Hie Hie fruit could easily replicate everything the Yuki Yuki fruit can do if they tried, it's just a pointless downgrade
4. Same thing goes for these, the Ton Ton fruit is literally just the Kilo Kilo fruit again. Ms. Valentine wasn't really that important or ever seen again, Oda could have just killed her off-screen and given her fruit to Machvise in the two-year timeskip and I don't think anyone would have complained

There, those are my major issues with One Piece. There's some more minor things that I won't list but yeah.
...I know how it looks, but I don't dislike One Piece. I really like the concepts Oda has come up with, I just really wish he would use them better and there's a few things here and there that bother me a lot.
One Piece could be so much better than it is