
The premise is simple enough. The main character seeks to become the pirate king by finding a treasure called One Piece (you wonder where they come up with these English names ..) and starts by assembling a crew. The pattern then becomes sailing from island to island, finding problematic villains and sending them packing, gathering crew members, and meeting people.
I had high expectations of One Piece. As one of the most popular and best-selling manga stories in Japan at the time (and still is) and of the shounen genre that I am fond of, I expected nothing less. So when Kaizoku-fansubs first started releasing good quality fansubs in 2003 (as compared to the HK rips, where calling the subtitles English is generous), I decided to give the series a shot.
I was disappointed.
As I began watching One Piece, it wasn't as though I didn't like the series at all. It was watchable. Generally enjoyable. It just failed to meet the expectations I had. That was probably the problem.
I continued to watch the series on and off, with the early main characters (Luffy and Zoro) growing on me slowly, and Usopp knocking down my interest a bit. I started warming up to the series during Sanji's arc. Finally, Nami's arc sealed the deal. For those familiar with her arc, which I still feel is the best of the crew's back stories, I can say there were multiple, very memorable moments. The feelings were real. I cried for the first time while watching this series. I cared very much for the characters, and for what happened to them. I went from lukewarm to obsessed about One Piece.
Its been over 5 years since then, and I still consider One Piece to be one of my favorite stories, both the manga and anime adaptations. I think One Piece achieves something very rare in its genre. This isn't something that I felt all along, but as I've examined why I continue to love One Piece, it's a theory that I think helps to explain it. Shounen series in general place a heavy emphasis on fighting. It defines the genre. Of course, there are reasons the characters fight, and those reasons can often result in long expositions and flashbacks (in the middle of fights, unfortunately). One Piece is no different. How One Piece is different is that the fighting is truly a means to an end, not because you know it should be, but because it really is (with many other series, the fighting is usually why we're watching, whatever else the story might say). Often, the aftermath and outcomes of the fights are far more rewarding and what one looks forward to, rather than the fights themselves. A subtle difference, but it makes all the difference in setting One Piece apart from most shounen series.

One Piece is at its most heartwarming in the aftermath of the battles, and in exploring its characters. It is most enjoyable when it focuses on the antics of the characters as they explore the places they go and generally get themselves into situations. The interactions between the crew members have spawned tons of hilarious exchanges and scenarios, many which are simply priceless. One Piece's main characters, the Straw Hat pirates, are molded after common cliches, yet, as with better stories, make those cliches their own and find a way to be quirkier. The characters, one and all, can be unabashedly silly yet transition convincingly to deadly serious. No transition is greater than with Luffy, the main character. Although he appears goofy and simple, his attitude is better described as fun-loving and child-like, with an innate fascination of the world, new things, and exploration. He isn't dumb; rather he ignores typical adult concerns and conventions because they just don't interest him. Yet we understand what matters to him. So when those things are threatened, his seriousness is wholly believable, even if it's in direct contrast with his usual demeanor. And when he is serious, he can only be described as incredibly cool ^_^
Remarkably, One Piece manages to balance a genuine lightheartedness through its humor and exuberance, with the gravity of the battles and hardships. That lightheartedness is missing from most shounen series. Other series have humor but there is joy in One Piece, which is apparently more difficult to create. Not to say that this necessarily makes a good story, but it is unique and I've seldom encountered other shounen series that exude the same fun and silliness, even as it packs on fights, special powers, and bigger and badder enemies.

I should note that One Piece lacks some of what I consider to be vital elements of a good story, most notably character development and growth, and real tension (i.e., death is possible). I'll skip the last, since most series aren't going to kill off their main characters, but I've not missed the other things, yet. First and foremost, One Piece is a lot of fun. It's a story that inspires imagination, and a desire to explore and see new things. It consistently makes me smile. And while real tension may be lacking, it doesn't lack for touching or heartbreaking storylines. One Piece is an amazing ride. Not for everyone, perhaps, but if themes of friendship, dreams, and exploration, wrapped in a fun package not afraid to be silly, with a healthy dash of fighting, appeals to you, what are you waiting for?