1. Oda is dedicated to his craft. He's been working almost non-stop for the last 28 years. Putting out this manga almost every week. I don't care who you are, that's impressive.
2. This is one of the most fleshed-out worlds in all of fiction. I mean, given the length, I would hope so. But, it just has a depth of characters and events that's hard to find a match for. It's a playground that you could tell just about any story in, as long as you find the right island to tell it on.
3. Oda is a master at getting us to care about characters. He's a master of giving us interesting backstories. Just look at what Kuma's backstory did for that character's popularity!
4. The art style is completely unique and different from any other shonen manga out there. And it instantly makes One Piece recognizable at a glance.
5. The manga is somehow still capable of finding ways to surprise us. Whether it's big new reveals. Or plot twists we don't see coming. A lot of manga authors get stuck doing the same formula over and over again. And Oda's guilty of that, too. But, Oda is a master of letting the formula work FOR him, instead of against him. He always finds ways to twist the formula JUST enough to keep things interesting.
6. Keeping track of this many plot threads and story lines is impressive. Oda must have a impressive notebook somewhere or a team backing him up. Because a lesser author would forget or abandon certain plotlines, if a manga went this long. But, most of the time, Oda finds ways to bring back plot threads from DECADES ago and use them in interesting ways.
7. All the mysteries and plot threads make this series REALLY fun to speculate about. Because there's still so many ways to combine and intertwine all these ideas that Oda's introduced. With other manga, the story goes in pretty predictable ways. But, it's still kind of hard to see where Oda is going with all this.
8. This manga can be really creative. Whether it's fun new Devil Fruits, or completely bonkers new designs for characters. Even the fights don't always go the way you might expect.
9. Answering fan questions in the SBS. I can't think of many other manga where the author basically gives us entire side-stories in the supplemental material. It just goes to show how rich the world is that he has so many stories that he doesn't even have time to actually draw.
10. Oda is a master at both planning AND improvisation. Oda gets a lot of credit for planning things out since the very beginning. And he deserves a lot of praise for that. More manga authors could stand to plan things out more than they do. But, he's also great at just making things up on the fly and making it feel like it was planned from the beginning. The Worst Generation were famously created in the span of a afternoon. That's just crazy! Oda knows when he needs to plan, and when he just needs to let his creative juices fly free.
11. The Cover Stories. This is something that I don't think I've seen any other author do. But, given us a sequential story for one page, that runs alongside the main story, was kind of genius. It allows you to do so much more, and cover more events, without the work and time it would take to make a whole side story manga. Plus, he takes fan requests for some of them! And that's just cool! It's kind of a holdover from the OLD days, where fans could send in character designs to Kinnikuman and see them drawn by the actual author of the story you love. It's a clever thing he did, and I'd actually like to see more manga do something similar.
12. The story is a lot deeper than most people expect at first glance. We're dealing with a lot of heavy topics in this manga! There's some actual philosophical and moral questions being raised in this manga! For a "silly pirate manga", you could get some real heated discussions about the nature of life, the universe, and everything!
