Okay, first of all: I love One Piece. Like many others that have rated it 5 stars (as they should), I too was enraptured by it. From the endearing characters, beautiful soundtracks and wonderful animation to the compelling story, it's one of, if not the, best anime I've personally seen. Like, high up there with Fullmetal Alchemist Brotherhood.
Of course, being a shōnen it's mainly directed at teenage boys, but has managed to gather a huge fanbase beyond the target audience, and I think that speaks volumes on its awesomeness.
And let me say this, for people who complain about the filler episodes: comparing it to Naruto and Shippuden, One Piece has 90 filler episodes out of 973 total episodes (as of 06/25/2021), making up 10% of the whole show; while the other two anime have 90 and 209 filler episodes out of 200 and 500 total episodes each, making up 41% of each of the two shows. That means that both Naruto and Shippuden spend more time on useless things than building up the characters and fleshing out the story. So next time someone complains about One Piece having "too many fillers", reminder them (or yourself) that 10% of an anime with 900+ episodes is absolutely nothing. Besides, you can always skip them, although, by the time of the first fillers you will probably be too invested to care.
Then comes the fact that it's way too long, and I agree, 900+ episodes are a lot. But by the time you're at the Syrup Village arc, or at the latest at the Arlong Park arc, you probably stopped caring about the length and begun to believe in Luffy as viscerally as his crewmates do.
And the characters are all beautifully written and 100% different from each other in terms of personality and physiques. And while, yes, both Nami and Robin have identical forms, you can distinguish them by personality alone! Which is way better than having them be different physically but revolving around the male characters and having no personality.
But the most breath-taking thing of all about One Piece is the story. It begins so simply (a boy with strange powers comes out of a barrel and ends up helping and saving a pink-haired, timid little guy from the big bad woman who had him scared to the bone) and expands impossibly wide, with substories that happen in the meantime and have real consequences, both direct and indirect to the main protagonists (see Ace's whole ordeal with Blackbeard and the Revolutionary Army).
All in all, despite what people think, One Piece is a wonderful anime that touches on many important subjects (other than the power of friendship, of course) and should be praised much more.