Its interesting just how many of the complaints are from the last 4 months or so.
Is it Oda slipping, or is it the audience being more picky with the recent week-to-week, and they'll be fine with most of these things once the full arc is done and we've seen the planned payoff, moved on, and we're in the next area?
And of course, the initial NW is weaker than promised… they spent 2 years training after struggling and barely surviving for two arcs and then having their asses completely and utterly destroyed at Sabondy. That training would have been a complete waste of narrative if they just instantly got their butts kicked again... they had to be monsters again for a little while like they were originally... but they're already hitting enemies at their level on the second island. But we've already seen inklings and hints of scarier things to come... crazy islands, weather, big monsters, and of course the emperors. (like TLC said, in two arcs they've had to go against a volcano, an underwater tornado, an assortment of dangerous deep sea creatures, a sea of flames, killer bull sharks and a dragon. and we've seen glimpses of the paths not-travelled like lightning island and gravity island. Willfully ignoring those things doesn't mean they aren't there.)
Yes, its dissapointing when Oda skips right through a fight... because it used to be he'd show those things in their entirety at length... but then we also end up in situations where we spend an entire extra year in an arc just getting one on one insignificant fight scenes, like in Skypeia, or Enies Lobby. I agree that the Thriller Bark general zombies getting stomped was a waste, but that was almost certainly a pacing thing, and Oda realizing after setting that up, that they'd be there for another half year if they ended up fighting all those cool guys. We're just now getting to the important fight territory, not a lot of reason to waste time on the early setup fights before the real consequences and danger have set in.
I'd rather not repeat EL's year long 3 volume fight sequence of every strawhat getting an equal one on one three chapter fight anytime soon. (And if the anime was doing its job, THEY'D fill in the fights, its a great filler opportunity, but of course they suck and don't take advantage.)
(As for not seeing the Fishman Island dojo, I was relieved on that account once we saw Hack and Koala and it became clear that any significant sights or characters that we would have seen there are going to be later, and there would have been nothing storywise to see there... just how the building looks. Koala the trained human in Fishman Karate is more interesting than any regular fishman would be... we'll possibly get some of it when we see Jinbe's final flashback eventually or catch up more with Sabo and Koala. It's hard to believe that Oda not only skipped showing the landmark, but also that didn't get a TON of people asking about the dojo and hasn't adressed it in an SBS without some reason to hold off.)
Some of these wasted opportunities are things long gone... but some of them are things Oda's still holding in reserve for pacing.
When Luffy was poisoned by Magellan at Impel Down, he endured hours of sufferings and survived only thanks to his will. From a storytelling standpoint, I think it was the best moment to give us Luffy's flashback (the one with Ace and Sabo).
Luffy on his deathbed, then you stop and go into a flashback for 8 chapters, that has no major connection to the location or villain at the time? That would have destroyed the pace and tension of the arc entirely. A better spot would have been right after Impel down on the way to the war… because that would have given us a stronger connection to Ace and Luffy's determination... but that has the same problem, and either way it would have blown the Ace is Roger's son and not really Luffy's brother bit, AND we would have been waiting on Sabo to show the whole time. That was a hard flashback to position for a lot of reasons.
Oda plops in flashbacks in the middle of arcs all the time, but they usually reinforce the narrative of that specific area. Suddenly going back to Luffy's hometown would have just thrown things off a lot.
(Where it was actually placed let us feel stronger for Ace retroactively, and makes things better on repeat viewings.)