I think a lot of people who say that they hate One Piece have also never read the manga. In fact, I think that a lot of people on /a/ form opinions based on what is the accepted status quo rather than from their own experience.
Don't get me wrong, I know there are people out there who genuinely can't stand One Piece, and that's fine.
i do find it annoying that /a/ and 4chan in general, despite supposedly being the wild free-thinking part of the internet seems to form a standard opinion about anything and then castrated anyone who deviates from it. It also bugs me about how people are determined to sage-bomb any thread on a subject they don't like. Some people seem to get offended at anyone having a shounen discussion for example. I mean, I don't like Lucky Star, but I don't go into those threads just to be an as, although some people do, so it isn't just shounen that the problem is with.
I guess that's one of the disadvantages of having an anonymous message board with few rules. You have to take the rough with the smooth I suppose.