Some examples I can think of right now:
-Napoleon did nothing during Big Mom's amnesia because it's shy. How convenient...
-Sanji and company found Kin'emon and the others "somehow", no detailed explanation given.
-Law being magically cured of his disease because of eating the Ope Ope in spite of giving no further explanation on how this happened or why Law could cure something so deadly, in its later stage and being just a kid doctor and amateur fruit user before it killed him.
-It remains unexplained how Moriah travelled with Thriller Bark to Paradise or how Krieg came back from it.
-Nobody ever addresses anything of importance. Robin spent two years with the revolutionaries but said nothing about it, Luffy knew Linlin was in Wano yet this was never touched upon, etc. Sometimes it feels like the characters don't share information in a natural way because it would negatively affect the plot Oda is preparing, but it makes no sense for Robin to know Sabo and keep it a secret, for example.
-Sanji wanting to save his family's lives. Overall, the whole narrative of Sanji being the kindest man we were given in Whole Cake Island felt completely lazy as forced to make the Vinsmoke's plot feasible. Sanji is the kindest when feeding people, but that's it; he had no problem with leaving a wounded Wiper untreated, nor gave a damn about the kids in Punk Hazard at first, and was okay with letting the minks die in advance of their potential threat to the group. He's a strategist, even machiavellian thinker, but certainly not the kindest, so I see no reason why he would want to save the sociopaths of his relatives.
-The Straw Hats fighting styles overall except for Luffy to a certain degree and maybe Brook because of the soul music.
-Hiyori giving Enma to a stranger like Zoro.
-The whole prophecy thing is becoming more and more lazy as chapters pass. No proper, well-written prophecy explicitly states things like "pirates will come to the New World to do this". That's not a prophecy, that's a spoiler.
-For an arc foreshadowed for so long, Wano's plot, and specifically that of the Kozuki clan, is a mess. It was first miscalled "Kogetsu"; then Kin'emon needed to formally introduce himself as a retainer and show his tattooed back to his later-to-be revealed close friends Inuarashi and Nekomamushi;, then the minks postrated themselves in spite of being as much of a retainer as Kanjuro and Kin and referred to Raizou with the "-dono" particle, which is a bit formal and the retainers never used it to speak to themselves (if I recall correctly) but to less-close acquaintances; then it is stated that Kaido was after Oden because of his knowledge of Laugh Tale yet this is never addressed during the flashback; then Oden is set to be executed yet is allowed to keep his swords inside the cage, and not only that, but Toki is able to freely approach him even though that same cage was shown to be guarded by Orochi's men previously; and well, I don't need to talk about the whole Whitebeard situation we all know.
...Yeaaahh I am gna need a detailed explanation on that one
