I LOVE OP, but why does it not garner the same type of hate as a popular series is sometimes surprising. The most I hear from people who don't like it is that they are turned off by the art, which actually fits the series perfectly IMO. But nothing else can be said against it it seems.
Isn't the answer simply? People who complain so much about OP's art (like me

) won't have read deep enough into the story to actually be able to complain about the plot
From the little I know about OP, I can also tell you that I find the general setting and story as well as a lot of the minor characters I see here and there, extremely off. It doesn't appeal to me in the least.
As for the 'hatred', that comes mostly with a fanbase which makes a good victim for hate. Very few people actually hate a certain manga, they mostly had the fan behavior (good example: boys love manga - maybe 3 out of 10 complainers have ever read one and just a single one out of these three bothered to get a recommendation first and not just downloaded some trash oneshot for better complains).
Our mod job here at MH shows already that the OP section is far more easy to handle than the Naruto section. While our Naruto mods cry every week over the bunch of deleted posts and warnings, the OP mods tend to relax and laugh in the opposite corner.
Now why? Maybe Naruto is easier to 'get in' due to better art and beginning story? Maybe the characters appeal better for fanfics and artworks which get people to try it out? Or maybe Naruto has a better (fan) promotion? Most through and through 'Narutards' have never tried any other manga from what I hear in the forums, so they're extremely focused and protective on it. On the other hand, most OP readers seem to be at least familiar with a handful of well known series.