Read on Twitter
The first chapter often isn't appreciated especially because it wasn't the first episode of the anime so I wanted to point out why it was so crucial to one piece and hopefully this will lead to more threads about why the start of one piece is good.
In Japan at the time, pirates were seen as bad people so Oda would need to deal with this issue. This is problem is dealt with in a few ways throughout the first few chapters.
First, we are introduced to Luffy, who is the audience surrogate in how amazing all of this new information is. Through his child like view of the world we are also spoken to through him.
From there, we are introduced to shanks who acts as the ideal version of what a pirate is. Through how awesome and free he is, it makes pirates look like an awesome thing. But people can be cool and evil so the next page builds upon that.
The pirates explain how to be a pirate is the sheer pinnacle of freedom and happiness and although this is from there view, these few panels really humanise the characters.
But Oda makes sure to highlight that while a pirate life is happy and free it's also dangerous and isn't something to be taken lightly. Only a few pages and we already have a good idea of a pirate while being a great first chapter that gives natural exposition.
But even through all of this, pirates rape, pillage and kill right? This is where Oda brings the bandits into the story. By using the juxtaposition of how terrible the bandits are to how good the pirates are it seperates them drastically.
Not just that but the majority of the bandits are faceless which makes dehumanises and continues to seperate them. With the only one of notise being absolutely awful.
Pirates are generous while pirates spit at the hand that feeds them. But when being spit at, a pirate does not fight back unless needed to. This is creates tension, a ripple in beliefs, as pirates seem to be lesser to bandits and is shown through luffy's anger.
After Luffy is beaten by them, we see what a pirate truly is. This still makes my hairs stand reading this. We see how a pirate isn't some Saint. They might let you do something to them but harming a friend is where they can't stand back.
Not just that, but since this is a battle manga we also see them destroying the bandits casually just to show that they could have destroyed them but they chose not to.
One of the most controversial scenes in one piece. Shanks sacrifices his arm. This can be interpreted a million ways but I see it as the ultimate pinnacle of your friends nattering more then yourself.
Through all of this we the audience get a very clear image of pirates. While some problems still exist, it's established a pirate is free, happy and fights for their friends no matter what.
And just to clear up misconceptions, Luffy takes out that same sea monster with a punch to show that shanks allowed his arm to be taken to give Luffy a life lesson. Also its a really cool spread 
