I've thought about this question a LOT but I think my most honest answer is "I don't know why I love One Piece". Like, I associate this question with trying to sell this series to someone else who perhaps was asking me what I found appealing about it, but I remember absolutely loving it as a kid watching it religiously on TV, and being amazed at how much I was loving it as a teen who would watch it ~12 hours a day for 2 months straight (from start to finish). I loved it from the first scene of two dudes sitting on a boat pulling in a barrel, until the last scene we saw today, and I don't know what magical element pulled me in in the way it did. In fact, I don't know that it's possible to fully understand it either; other Shonen, while I love them, haven't affected me quite as strongly as One Piece has.
So I'll just list the things I DO love in the series instead
Let's start with the clear direction of the plot. It's not the characters chilling out waiting for bad things to happen, it's characters going from physical point A to physical point B and overcoming the obstacles that stand in their way. Luffy is going to be Pirate King. This means he goes the Grand Line. Once he reaches the Grand Line he learns that he needs to reach the last island - Raftel - to be Pirate King.
Next is the characters, obviously. Just about every big anime manages to nail its characters but so does One Piece, and One Piece has the unique task of juggling 9 (now 10) main characters simultaneously, which is nailed as far as I can tell. True, sometimes the characters sink into 2D caricatures of themselves (especially main man Franky :( ), but once the important scenes come up we feel the full force of their personalities shine through.
I think the main one for me is the world-building. We understand all the major (and most of the minor) powers at play in the OP world. We understand geographically the way the world is laid out, and more impressively, we see intricately how the geography of the OP world is tied to the power-struggles. We understand why major events are major, and the way the world-building is tied to the plot is done masterfully - far better than in any of One Piece's counterparts (the few that I know anyway).
Last thing I'll mention is the Devil Fruit system. It's cool and handled well in terms of balancing them...
<Help I went on another Haki rant I really need to see a professional>
...HOWEVER it is sadly offset by the fact that One Piece absolutely suffers from the typical Shonen nonsense where we actually have no idea how much a punch/attack from a strong character will hurt another strong character, and if the brute force aspect was ever taken even a little bit seriously, our minds would break from how little sense it made. Some attacks are revealed to be so strong that they'd annihilate a mountain, but when used against a strong character made of flesh and blood (chemically weaker than mountains), it hardly makes them shrug. And then when you throw Haki into the mix, everything just gets worse. Haki supposedly made Luffy go from utterly unable to 1v1 a Pacifista to able to take it down with a single hit. That's an order of magnitude that must be near the hundreds (probably even more - a Haki-less Jet Gatling hardly did anything to the giant cyborg). This means that the incredible feats of strength that Luffy had shown before the time-skip would now be hundreds of times easier to accomplish with Haki. So it's not impossible to accept something like "if Luffy had Haki, he could probably could have one-shot the likes of Lucci, Crocodile and Enel", but then we consider like the destruction of Arlong Park with one kick, or Luffy punching through the floor of that cavern in Alabasta as he finished off Crocodile getting 100x stronger, and we wonder that Luffy doesn't have near-Godlike strength, especially against Haki-less entities.