I've noticed that the sagas of the series tend to alternate between light-hearted adventurey storylines that only raise the stakes towards the end and more serious, plot driven stories which are more serious and more directly involve the Marines and the World Government and their corrupt nature. I'm getting my info on the division of the story into sagas via the Japanese Shonen Jump website for the series, though that only covers up to the Marineford War saga, so the Fishman Island Saga and Pirate Alliance Sagas are speculative in terms of official names and start and end points. For them, I'm following the wiki. While it's not always accurate, the info it has on these sagas seems reasonable to assume. I prefer to work with sagas instead of arcs when I'm talking about One Piece's story structure, since most arcs, while containing their own beginnings, middles and ends, are still just small contributions to these larger storylines that feel more full and complete. Each saga is more than the sum of its arcs.
For the sake of proving my point I'm gonna run through the series to date one story at a time.
First up we have the East Blue Saga (EBS), which is an adventure storyline. Luffy travels from place to place, keeping it very carefree most of the time, and gathers his crew one person at a time. It's all about discovering new places and meeting new people. It's only at the end, when Arlong is finally revealed, that a real challenge is posed to the crew. While the marines feature, they're pretty tame in EBS and the corrupt sailors like Captain Morgan and Captain Nezumi seem to be a minority.
The Baroque Works Saga (BWS) is a plot and action driven storyline. Unlike how Arlong only appeared in East Blue's final quarter, the Baroque Works organisation threatens the crew nearly from the start of BWS and their presence hangs over every individual arc of the story. While the marines are still ultimately a positive force here under the command of Captain Smoker, the World Government's issues start to come to light, as we see how Sir Crocodile has abused the Royal Warlord system to attack Alabasta with impunity.
Skypiea Saga (SPS) takes things back in an adventurey direction, with the marines and the government hardly featuring after the start of the saga. The focus is placed more on discovering the fantastical world of Skypiea. While end boss Enel has a presence in the story from early on, SPS gives another example of the story escalating fast and late as he moves from impeding the crew's treasure hunt to threatening the entire island.
We swing back to the World Government in the Water Seven Saga (WSS) and get a good hard look at the genocidal lengths they'll go to to protect their secrets. The crew clashes openly with marines and government agents and a war is declared between the Strawhats and the world. Once again the atmosphere is far more threatening from the get go. We know that Kuzan is nearby and that he could annihilate the crew in a heartbeat if he wanted. Even before CP9 is revealed the threat of crew members leaving or being lost is an everpresent theme, from having to wager them in the Davy Back Fight to Robin and Usopp leaving of their own accord a little later.
In the Thriller Bark Saga (TBS) the crew goes on a more laidback journey through a supposedly haunted stretch of sea. It's light, full of jokes and references to classic horror movies both in its visuals and content. Even though Moria is introduced early on, TBS doesn't really get serious until late in the game when Kuma appears, and when the sun starts to rise during Luffy and Moria's final clash. The serious plot development is all packed tightly into the saga's final chapters.
The Marineford War Saga (MWS) is of course an action and plot part of the series, with higher emotional stakes than WSS and the guts to actually follow through on the threats WSS made - those being the loss of crew members and friends through separation or death. Once again open conflict with marines and government men is a thing that happens, and the status quo of the series is shaken up hard.
Then, after the timeskip we have the Fishman Island Saga (FIS) which early on deals with the crew's reunion and resolving the long-running plot threads regarding Jinbe and the Sun Pirates and the racial conflict between humans and fishmen. FIS escalates rapidly near the end, after such a lighthearted and exposition heavy start, when Hody finds a way to actually threaten the destruction of the entire island.
Finally, the Pirate Alliance Saga (PAS), presently running, gets the plot moving again. After a slow lead-in on Punk Hazard its revealed that the crew is pitting themselves against a Royal Warlord, and the ripple effect of the confrontation will reach Emperors and shake the power balance of the World Government, the Warlords and the Five Emperors. The issues with the Warlord system are demonstrated again and discussed openly. I think, based on what we've seen so far, then end of PAS will bring some big changes to the story.
So what does this all mean? Well, it can be a good indicator of different tastes from what I've seen online. A lot of people gravitate towards either the adventurey sagas or the plot-heavy sagas exclusively and dislike the ones of the other kind. Some people, like myself, enjoy both kinds of stories. We can also make predictions about future developments based on this analysis of the series' structure. Let's assume that whatever comes after Dressrosa will move back to a lighter, less full-on tone. Zou and Wano (likely both of them, as part of a single new saga, I would guess) are good candidates for this, being relatively unknown places to explore and seemingly unlikely to have any big name figures on them. The conflicts the crew has kicked up with Big Mum and Kaido will probably have to wait til the next plot/action storyline.