This manga is not the best because of blindness of some jerkish fans, it is the best because of it's unique and highly engaging plot. You cannot even think how the author can surprise you and how the small details can actually make you go numb. When I re-read the series what I saw was that there was some things or some details at the end of each arc which were hard to notice and those minute details went on to make a big and absolutely mesmerizing arc.
Now, moving on. About the characters. The characters all have a very distinctive art with accentuated features. It may look a little odd to begin with but the art slowly grows on yourself. The background art is detailed but I have seen better in many other series. The main thing about One Piece is the main character. I have not seen such a protagonist in any series. Oda makes you love the main character so much that though him being absolutely dumb you hoot for him all of the times.
What many point out is that Luffy does not goes any character development in the whole 700 chapters. Well, I'd say he does undergoes a character development but what makes the main character of this series unique is that his never changing character. Ask any One Piece fan, Luffy changing his attitude or any trait is simply beyond imagination and that is what Oda has sown into the minds of us readers that the main character, no matter what, will not change. As per the character relationships, you'll be damned if you get some better friendship and comradeship views in any other series. The drama, the backstory of every character in the crew of Luffy is just heart touching.
Coming on to the fights, the biggest reason why most of the fantasy and supernatural genre people don't like One Piece is that there no super mode and unbelievable battles in the series. Everything is stitched nicely with the " One Piece Logic ". What amazes me is the randomness of the fights, there is no particular form of fighting that anyone follows. Especially the main character, there is no uniform style. And this is what makes the One Piece fights natural where anything can happen. Also, the addition of the absolutely amazing concept of " Devil Fruits " in the One Piece universe serves as most entertaining element. No matter how goofy people think Luffy looks by stretching his body, believe me when you understand the basics of One Piece it would look pretty damn badass. Also what I like thoroughly about One Piece is that no " Instant Power Ups " are given in the series. Here, the strength either comes from training or devil fruit. The main character has the lowest tier of the ability and there are hundreds of people who could defeat him in a punch. But that's what makes it good and worth reading.
And the last reason why One Piece is the best. The well timed time-skip. I don't think any series has a better time-skip situation than One Piece. When the time-skip gets over, there are just so many things to look forward to that you cannot even count. Even while One Piece has rushed to 707th chapter there are many things we don't know. Time-skip has made everything more interesting, new characters are well placed by Oda and pace of the arcs have been better since then. Also, the coming back of many characters has been the specialty of One Piece. What you may judge has irrelevant and not important could be the game changer.
I think this must have cleared some doubts of the people not liking the series. Unless you give it a try you won't be in a position to judge it and I'am pretty bold to say it that there is no long-term fantasy series as good as this.