Really... This here...

I don't know what seriously bad can happen in OP so that I would quit it.
The things we watched so far were only better. And I think that Oda's plan is, actually, to make this story based on slow gradation and that's the secret of this great anime.
We watched, how many episodes? 640 or sth, right? But no matter on which point of the story you were you could only say: Now story is very good but knowing what will happen soon... Man, this will be even better!
And really, when Luffy started gathering the crew you knew that the crew would have to go through many adventures and you were like: Hell! I can't wait! And when they reached Grand Line you knew that the crew was going to have harder moments and more interesting fights and whatnot and even now when they are just one step on the New World you can't wait to see new arc because you keep saying: Damn, what will happen with Yonko? What will happen with Akainu? will Luffy meet Shanks... and so on and so on...
And Oda really knows to make an arc to be like a whole ****ing different anime, but anime which somehow is still OP and of style, with so many details, with so many real life things... And in OP you don't have these black and white things, people... Hell, try to classify who are villains in OP and you'll see what I am talking about. Like Doffy said:“Pirates are evil? The Marines are righteous? These terms have always changed throughout the course of history! Kids who have never seen peace and kids who have never seen war have different values! Those who stand at the top determine what's wrong and what's right! This very place is neutral ground! Justice will prevail, you say? But of course it will! Whoever wins this war becomes justice!”
And when someone who doesn't even watch OP says why do you like it you can't just explain that. You have that special thing with OP no matter do you watch anime for I don't know, years or only months...
And precisely because of that OP is definitively not an anime/manga for quitting!!!