To me, Wano had three main issues going against it;
1) Inability to deliver on hype.
2) Story/character bloat.
3) Atrocious pacing.
The inability to deliver on the roughly decade long hype train that Oda established far in advance gave the whole arc a crushing aura of disappointment. This was Oda's first major mistake. Openly promising to make Wano greater than Marineford was dangerous advertising on his part. This is the more minor of the three issues however.
The second major mistake was bloat. Perhaps Oda's greatest weakness as a writer is his self-indulgence and lack of self control. Oda will indulge in whatever whim takes his fancy. This usually manifests in Oda creating about twice as many characters than necessary and then having to go out of his way to find something for all of these extra characters to do.
The aforementioned bloat then severely affects the pacing, the final and most crippling mistake.
Wano's pacing is among the very worst that I have ever read in a major shonen arc. Wano manages to be both snail paced
and rushed which sounds paradoxical at first but is rather simple to explain.
Oda creates too many characters, this causes bloat. This bloat slows the story to a crawl as Oda has to keep track of twice as many characters as necessary all while finding things for all of these characters to do. He focuses on this and before even Oda knows, we are over 60 chapters in (the length of whole Pre-TS arcs and more chapters than Marineford itself) and still aren't near having a battle. Oda panics, we get battle, it is rushed and lacklustre, Oda is still having to deal with a bloated cast, whole arc spirals out of control and the payoff (10 years of expectation) is a complete bomb.
The pacing is what kills Wano's entertainment factor more than anything. To put into perspective, Wano
Arc is over 150+ chapters long while the entire Summit War
Saga was 108 chapters long.
The faster a flawed arc is, the less time readers have to notice mistakes especially if the fast pacing is combined with flashy eye candy. The slower a flawed arc is, the more time readers have to notice said flaws. Wano is painstakingly slow and it's eye candy is painfully rushed.
Oda completely screwed up by dragging out things nobody wanted to see and rushing things that folk were excited for.
Wano is what happens when a storyteller overextends themselves. Oda is certainly a capable storyteller but he exceeded what he is capable of when attempting Wano. He was overly ambitious and overly indulgent.
If Oda had trimmed down the cast and made Wano 1/2 as long with a focus on the fights then he probably would have been on a winner but he didn't so here we are.
Not everything would have been fixed if Oda had followed the aforementioned suggestions (there would still be the usual illogical action or contrived scenario that typically comes with Oda's writing these days) but the problems that most crippled the entertainment value of Wano would have been avoided.
Well, those are my overall thoughts on what is my most disappointing entertainment experience to date. Thank you for the tag
@Pooth