It's not my first One Piece. I've watched the anime up to about Skypiea and read most of the rest of the series in the manga, (though I'm not really caught up nowadays). I don't think it matters either way though, an adaptation must be able to stand up on its own, it can't just all be fan service to the existing fans. They've changed a lot of things from the source material, which is usually not what fans like to see so I don't think they have only existing fans in mind when making this show.

In the manga, Reverse Mountain arc was like 5 chapters, and it was two full episodes in the anime.

I just checked the manga, Reverse Mountain not including Laboon was less than one chapter, from the middle of chapter 101 to the third page of 102. The only notable thing that happens is that they almost hit a pillar but Luffy balloons up to bounce off of it. This is translated to about 1:30 to 7:30, six minutes of LITERALLY NOTHING HAPPENING, we're just watching the crew struggle to hold on to something while the current carries them. Oda would never.

I honestly cannot judge if the pacing is poor, because my experience with the anime has skewed my perception of time.

Yeah, the anime is infamous for how abhorrently slow it is. It set a bad example and I'm sad the Netflix adaptation followed it, especially since the main reason the anime is slow is that it had to follow the pace of the manga for most of its runtime, which is not a limitation the Netflix adaptation has. So now it's just slow and boring for the sake of being slow.

The writers of the show have the advantage of foresight, so they know how moments in the current season will return later to be important. When Oda was writing the manga, he probably didn’t know that Laboom and the Rumbar Pirates would come back around to form the core of an emotional story. It was just the crew got swallowed by a whale and Luffy punched it until it joined the crew. The writers know now that having Luffy sing that song and doing that dance will strum the heartstrings of hardcore fans, but without that context, it just seems weird.

I don't know what you mean here. Oda absolutely knew and planned that Brook will join the straw hats when he first introduced Laboon. He just didn't plan for it to take so long to reach that point in the story. This long-term plot connections are one of the best parts of One Piece. These cheap attempts to pull at my heartstrings now are only going to lessen the emotional impact later when we meet Brook for real. In the anime and manga, the whole Laboon arc was plenty emotional and impactful, even without the foresight. If anything, this cheapens is.

If it helps, think of Luffy like an empathic id. He’s not smart, and he’s often oblivious to subtext, but he has a strong moral code and an absolute devotion to doing whatever he believes is the right thing. He can instantly read people and sincerely wants everyone to follow their dreams.

Right, but he never flips between angry and excited and back to angry at the flip of a switch. If he's angry, he stays angry for a while! And determined. I see zero determination in this Luffy. He's a badly written bipolar disorder.

In the manga, instead of singing Binks Brew, Luffy challenges Laboon to a fight, declaring them rivals and demanding that Laboon be ready for their next fight. Giving the whale a purpose ends the whale’s despair and depression. It’s a classic Luffy solution, in which punching your problems is actually a deeply intuitive understanding of what someone actually needs. This is a common theme, therapeutic punching stuff.

Singing to Laboon instead of dueling him is a controversial change, but I liked it. Luffy recognized that Laboon didn’t need a rival, he needs compatriots. He misses his friends, and would kill himself trying to see them again. By singing and dancing, he helps Laboon understand that when old friends leave (and turn into walking skeletons) new friends can help with the sadness and longing.

From a high level view, I like this change too. In the moment, part of me was screaming "this is not how it happaned!" but this is an adaptation, not a 100% faithful retelling, so I can happily accept any changes as long as they make for a good show.

The problem is that the execution was just terrible. The minute-to-minute and scene-to-scene writing is just awful and terrible. I don't know how else to describe it. So the high-level "Luffy joined him into the crew instead of declaring a rivalry" is good, the low level details of how it was done are just unacceptably bad.

[..] But you only have that context like 300 chapters later, so in a vacuum it’s just a random choice to sing and dance at the enemy.

A scene in season 2 should feel natural in season 2, not only with hindsight in season 10. One Piece (and any other good work of fiction) never pulls things like this. And even if it was true, are you saying that Luffy is essentially telling the future? Just how far do you think his gut can take him?

If you are seeing it for the first time, you sort of just have to strap in and accept everything that happens at face value. Trust in Luffy, and things will mostly work out. In fact, I think that’s essentially what Nami says in a later episode to another new friend. Don’t expect to know what Luffy will do, but know that it will probably make sense later.

Yeah, I did see the part where she says that (it's to Vivi in Little Garden, before she goes back to Usopp). It's still not even remotely close to how the manga/anime dynamic works.

It’s a good show. Like, objectively, reviews are nearly unanimously positive.

If you like it, I don't want to take that away from you. I am giving my own review, as honestly as I possibly can. We are discussing it here, and I do enjoy this discussion itself. So I don't think "other reviews seem to like it" is really relevant here.

I guess if what counts as a "good show" is a show that a lot of people like, then yeah, it's a good show. But then so is a lot of reality TV. I think a good show is a show that has something to say, that takes itself seriously where it matters, that doesn't constantly take me out of my suspension of disbelief. And I don't think this is a good show.