So what do you think draws the line between actual criticism and people who are just simply bitching and complaining?
Knowing your stuff and giving specific reasonings showing it is a good way to support criticism. Such criticism is also more credible if it fits the overall tone of the work it's approaching; this is particularly common in One Piece (e.g., "we want more blood and violence in this cartoonish and soft story!"), and if your criticism doesn't even respect the basics of the work then you are asking for the moon and therefore complaining.
For example, I suggested that chapter 1000 would have been improved if Oda had visually explicited a parallelism between Luffy punching the sea king in chapter 1, Ace punching the dragon statue and ultimately Luffy punching Kaido (here in dragon form) as the three panels would be drawn in an almost identical way. Since all of this is indeed supported in proper reasoning (parallelism is a very powerful rhetorical tool for visual works) and it would fit One Piece (Oda has done this before; for example, when stressing the similarities between Doffy and Crocodile by drawing two of their techniques almost equally), I consider it to be criticism and not just complaining.
Criticism also needs to grasp what Oda intended to do with the decision you're questioning. If a person complains about Franky defeating Señor Pink with a rain of punches but never noticed that it was meant to refer the rain as a rhetorical elemment in the latter's past, then the complain lacks any validity.
There's also the, let's say, uncomfortable truth that some people are far from illiterates in certain areas involved in this story; and I say uncomfortable because, when you're among the ignorants on said areas, it is harder to even consider the possibility that there's more objectivity than it'd seem in artistic productions and some people may know their stuff as good or even better than Oda, who at the end of the day isn't a perfect creator nor has the time to review his own work in depth before releasing it (which is a very important part of artistic creation). This scenario is especially common in forums because they generate this illusion of "universal incompetence", which is, anonymity is confused with being a noname and people feel freer to label as "bitching" a totally valid criticism on the basis of assuming the person writing it wouldn't be able to come up with something better than Oda; which is delusional.
These are some opinions I have on this issue, anyways.
