Because the text got so long, I marked the key-elements in bold
As of June 2015, One Piece has broken the world record for Most copies published for the same comic book series by a single author… by selling over 320 million books. Even taking into account the fact that it had 77 volumes at the time, it’s proof of the huge following this manga has (even more if you take piracy into account too). If you’re one of the people I speak to from time to time, who ignored this series or have thrown it aside because you don’t like the artstyle, because you think it’s “just another shounen”, or because you think it’s just too damn long (or if you ever saw the 4kids dub and thought the anime was too childish - know that the Japanese version is already less impressive than the manga, and the 4kids dub was horrendously slaughtered by overdone censorship) : I would hereby urge you to think again. Try to imagine those three hundred and twenty million books, screaming at you that you should just join the group of people who read it… or perhaps I can convince you to read it, with this possibly-too-long text about the manga that manages to make my day whenever it comes out.
It’s quite impossible to summarize the storyline of this manga, but I will try to give a view on what you should and shouldn’t expect.
The first thing you should probably realize before you start reading One Piece, is that it is first and foremost an unpretentious “shounen” manga - meaning that there won’t be too much brutality, and the number of deaths is kept to the minimum that the author deems necessary to further to story. There are some prominent jokes like “nobody dies in One Piece” and “bullets only work in flashbacks” which are taken serious by some people, although the actual readers only say those ironically - even if there is some truth to these jokes, they aren’t entirely true either. And well, it’s not like a manga needs Akame ga Kill-levels of death to be good. One Piece knows it is a shounen series, and uses the tropes that come with it to it’s advantage.
The second thing you should know is that this manga is a “formulaic battle shounen” - the premise is that a boy eats the Rubber Rubber Fruit, becomes a rubber-man who can’t swim, and decides to become a pirate and find the biggest treasure in the world. Of course he will meet people on his travels, leading to friendships and fights on almost every island they land on. But of course, Luffy always wins, right? Right? Spoiler: he doesn’t.
If either of the above two points make you contort your face like someone shoved a turd into your nose: don’t read it. If you still think this manga might be okay, you should definitely give it a try, and I will try to explain why.
The first thing to point out is, of course, the simplicity of the premise. While a simple (or even nonexistent) story isn’t always bad, a more complex storyline is often preferred. While One Piece delivers by giving an outstanding execution for the “simple” storyline in the initial arcs, it also prepares certain plot points for the more “complex” (don’t worry, it’s not that hard that you won’t get what’s going on) story that becomes visible much later - some of the things that are talked about in the first 50 chapters are important for things that happen over 400 chapters later. Subtle hints for later events can be found everywhere if you know what you’re looking for, which makes even re-reading this series a pleasant journey!
As pointed out above: the execution of the story is outstanding too. What I mean by that is that the moments where One Piece gets boring are really rare: it’s either hilarious (which is one of the strongest points of the series, more below), exciting because of the ongoing battles, or exciting in an entirely different way: because of the revelations of things that are important to the overall plot. How many series have ever made your heart race after a chapter where all they did was talk? Because I sure can’t think of many other series that pulled that off for me.
This is of course accompanied by the pacing, which is pretty good. There was a period during the Dressrosa arc where some people started complaining that the story was going too slow, but that was mostly because of the breaks between chapters - I saw no problem with it when reading it in one go / by volume. On the contrary, some chapters are so filled to the brim with new, exciting content that I’ve often wondered how Oda manages to take so much good stuff and put it into one chapter, without making it feel overdone and without running out of ideas.
Speaking of ideas, Oda must have a lot of fantasy: the world of One Piece is huge, exciting and filled with all elements of fantasy and science fiction that you could ever hope to see: Dinosaurs? Giants? Giant goldfish? Cyborgs? Mechas? Zombies? Check, check, check, check, check and check. And there’s more where that comes from! Even the superpowers in this series are fantastic and original: every “ability” is unique, and fights are won by using your powers in the best and most original way possible. While physical strength is important, fights aren’t only won by “punching harder than the opponent”. Finding out what the opponent’s powers are and how to beat them is key to winning any battle (and unlike the antagonists in Bleach, those in One Piece don’t reveal it that easily). This makes for great fights, which leave the reader wondering who will win and how that person will finish the fight.
Oda’s fantasy also works wonders when it comes to plot-twists. It’s not exactly rare that fans come up with wild theories about what will come next - it happens in other fandoms too. However, while tons of those theories seem all too wild, Oda will always come up with something that tops even those.
There's also the matter of comedy: this manga often made me laugh out loud, and not just get a mere smile on my face - although I am now conditioned to have a smile on my face whenever I see that a new part has come out. And Oda even puts this into the fights - turning exciting battles into gut-busting moments of absurdity at the drop of a hat.
There are also deeper themes to One Piece, like the classic shounen theme of "protect/cherish your friends", but even more importantly in this series is the theme of "dreams", with "fighting for your dreams" and"the inheritance of dreams" as sub-themes. While "fighting for your dreams" is also prominent in other shounen series like Naruto, One Piece has a way of handling that theme that feels unique and is hard to put into words. Let's just say you have to experience it to understand what I mean :p
Last but not least, Oda alsos put in a lot of “extras” to enhance the experience: the cover-stories and the hidden Pandaman (and his friends) are some of them. Especially those cover-stories are sometimes important for the plot, or to check back on islands the crew left behind a long time ago.
The role of main character is given to Luffy, a laid-back, goofy and likeable guy who has the power of being made of rubber. An original power if I ever saw one, and combined with Luffy’s personality it’s a great recipe not only for some great comedy, but also for the previously mentioned great fights. Oda’s fantasy shines through Luffy’s power, whether he gets in trouble because he didn’t think too much before acting, or because Luffy found a new incredible way to become stronger and protect his friends. Luffy is one of the main reasons I kept reading this series when One Piece was still “simple”, and is definitely one of the most goofy protagonists I ever saw.
Next to Luffy shine his crewmates - a group of quirky and interesting people, who can come or go: not every crewmember is destined to be with them for the entire journey, and people can always join at later times. The backstories of Luffy’s friends are all gripping, and their often tragic pasts form a beautiful contrast with the laugh-inducing characters they become later on. The interactions of these colourful characters easily fill up any empty moment in the story - whether they're be heartwarming, funny or otherwise entertaining.
Of course there are also tons of other side characters, with varying degrees of memorability. Overall, the characters are all interesting, fleshed out to a degree, and - very important for the comedy - quirky. How well those characters are done can be best illustrated by the fact that lots of people had to cry at the death of a certain character - a character you would never expect to care for when you first meet him/her.
The artwork is the most frequent reason I hear whenever I ask people why they don't want to read One Piece. It's extravagant character designs, with often huge upper bodies and small legs, combined with the goofy facial expressions and overall "not great"ness (especiall in earlier chapters) have made lots of people tell me that they "can't take it seriously". While on one hand that may be true for some people, I never really had that problem, and once you get used to it that problem should fade away for most people. On the other hand there's the fact that this goofy artstyle works wonders to enhance the comedy. Oda's artwork also improves tremendously after the first 200 chapters.
Panels are also filled to the brim with tiny details (for example in crowds you can go look for the previously mentioned Pandaman), and the action scenes are fluid. Scene transitions are done in interesting ways as well.
Although One Piece is a battle shounen of gargantuan proportions, there are tons of fans who are still craving for more. A colourful cast, combined with great humor, epic battles and a creatively built world, tied together with the great execution for simple and more complex stories alike, make One Piece the most fun manga I've ever read. And believe me, I've been looking for better or equally good series for a long while now. It's worth every second spent reading it, which is saying a lot given the length of the series. Prepare your wallet, and start your journey!