Lets start with the good. One Piece is a series that is praised for its world building and rightly so. Its world is extremely diverse and well built that its always enjoyable to see a new island in every arc. I can assure you that it won't disappoint you in the adventure aspect. The plot and story is well intricated and beautifully built up in every arc, I personally feel that the story is the strongest part of the series (even more than the world building). It has a surprisingly dark setting for a series where you only expect lightheartedness. The character writing is a hit or miss, sometimes even the most minor characters get rich backstories but at times even major characters tend to lack development or even a simple backstory.
Contradictory to what many fans would say, One Piece is not perfection nor is it peak fiction. A common criticism given is that it is far too long. Though I disagree with it, it is worth noting that the pacing of the series gets quite slow in its longer arcs. This is more commonly an issue if you're a weekly reader but is not too noticeable when you binge-read the series. There are far bigger criticisms than this.
The biggest and most valid criticism about the series is the lack of character deaths. I won't state any character names to avoid giving spoilers but there are times when a character would be given an extremely emotional death that would have perfectly concluded the character's story...but then suddenly turn out alive the next chapter/arc. It's one of the most frustrating aspects of the series at times since there is usually no reason to even leave these characters alive (since they normally tend to be extremely minor characters anyway). Though in the rare instances when characters are killed off, it tends to be a perfect one.
Another criticism of the series is the handling of the development of the main cast. Each of the Straw Hats have a certain unique trait or charm in their personalities that tends to be influenced by an event in their backstories. They do develop as characters throughout the story but it's usually far too subtle. In hindsight, it sometimes seems like they barely even developed in the 1000 chapters of the series. Luffy is probably the biggest example of this. He is a seemingly stagnant character from start to finish and seems like he hardly even develops as a character at all.
However, It is arguably a good decision to keep the characters' development subtle. The main cast is lovable in their own right and changing them too much might take away what makes them so special in the first place. Do you ever wonder why Luffy is so well liked despite being a stagnant and seemingly generic protagonist? It's because of the fact that he's such a lovably generic protagonist that is put into such a dark setting like One Piece's world. His presence brings a smile on the faces of the gloomy and hopeless masses he saves in the setting and as a result makes you feel the same joy he does.
Finally, here is the one single aspect you need to keep in mind as it is this one thing that will make or break your love for the series. Comedy is and always will be a central aspect of the series. A thing that makes One Piece different from other shonens is its blatant implementation of humor in dark aspects of its story. I understand why it may be a turn off for many, since can you imagine a serious all out war being interrupted by a random gag? However, you will come to notice that this aspect is only ever present when Luffy is onscreen or involved. The One Piece world is dark, corrupt and filled with despair for the masses who are constantly oppressed and it's so refreshing to see a happy-go-lucky and optimistically childish character break that cycle of despair and bring joy in the hearts of the characters.
So to those who are expecting a dark story like Berserk or Attack on Titan, this series is not suited to you. Humor and gags are and always will be a part of the series. However, if you plan to approach it with an open mind and let the story do its thing, then I can assure all newcomers that you will witness a masterfully written story, a wonderfully crafted world and a cast of memorable and lovable characters that will take you on an adventure you will never forget. If you think that the good outweighs the bads I mentioned, then I can assure you that you will love this series as much as millions of others do.
