One Piece's world has already been praised, but I wanted to add another factor to that, which is the fact that One Piece's central tenant is to "always keep moving forward".
One Piece as a story is diametrically opposed to the ideas of stagnation and inaction. The characters in the story are not forced to do anything in the story, at least not in the traditional sense. Luffy does not have to find the One Piece or 'else', he wants to find it because he wants to, and that desire is what drives the story. In that vein, Luffy (and by extension, the story), hates it when people laze around, wait for something to solve the problem for them or take the easy way out. Look at every crewmember that Luffy finds along his journey, and see how all of them were trapped in situations that kept them in some sort of aimless, stagnant state that Luffy rescues them from.
And it's that sort of will that you see at the heart of most of the arcs in the story. The obstacles the strawhats meet along their journey are never world ending. In fact, they are almost always in a position where they can turn around and ignore the problem. It's the way Oda compartmentalizes the stakes of a conflict to be emotionally personal to someone in the story, not always the strawhats, and the fact that the protagonists choose to confront those issues that convince the audience that this is worth going through.
In that way, the story becomes a sort of parable, as most epics are. We choose to go on our respective journies in life, and we'll likely encounter a dozen things that will get in our way, that'll ask us to give up. One Piece says that the struggle is something to be admired because that's what makes the decision worth it.