The secret to one piece success is in its story telling. Many manga series become dull and lose their quality with each new arc, and the tale becomes too repetitive and boring to read.
In the case of one piece every new arc becomes bigger than previous arc.For example, people used to say that the Marineford arc was the best, but now they say that the Wano arc (current arc) will be the best. Even writer Eiichiro Oda stated in an interview that Wano will be bigger than the Marineford arc .
The vast world building is another great aspect of one piece.There is just so much in one piece. So many different types of people ,creatures and nations.
Then there are the mysteries One piece began on July 22,1997, and there is still a lot to learn about the story . Instead of solving anything, I feel like we're still piling up stuff we don't know, which makes it so much more exciting to read every week and unravel all the myteries.
The charaters of one piece are also well written as every one has a good charater development and every one of them have different goal but work together to achieve their goal.The side characters also feel like they have meaning unlike many manga's.
If you haven't read One piece just give it a try and you will not regret it even though it may feel like there is so much to cover.