If you allow One Piece to distract you with its art style, or the unusual characters, you miss the point. I understand that many of the characters are strange and confusing--why is that reindeer wearing a hat, why don't that man's hands ever leave his pockets, what's with the swan-wearing crossdresser? However, if one watches the show without falling prey to these inquiries, the film makes absolute sense.
You needn't be versed in Shakespeare to understand which character is the coward, which character is playing it cool despite being in love with another, and which character is the generally goofy but profoundly driven character. The urbane villain, the misguided rebel, and the doomed retainer...these are all atavistic tropes presented in One Piece just as they are in any other work, but with more swans.
The archetypal nature of the film extends to its plot as well. It's unnecessary to understand why the princess is on a pirate ship; it's unnecessary to comprehend why her most loyal guards can turn into a bird- and dog-man respectively. I don't mean for my readers to mistake my dismissal of the need to understand these things for a rejection of the film's value. It's profoundly the opposite.
The Japanese practice of producing OVAs (Original Video Animation) is something I've come to appreciate. An OVA is usually an addendum to or remix of a successful anime series; a way of saying either "And then this happened..." or "But things might have happened this way..." The most impressive task of OVAs is that they must cram a multi-episode, if not multi-season, storyline into a few brief minutes; even if they aren't theatrical releases, OVAs need to appeal to an audience that might not be well-versed in the background material. As such, OVAs are adept at communicating everything that the viewer needs to know about the actors, and situations, with which they are presented.
The characters are archetypes, the plot is eternal, the Pluton is just a MacGuffin. Once you understand that, One Piece ranks amongst the greatest films of all time...because it's functionally identical to them along all the valences that matter.