Some things I love about One Piece:
The main characters are not brought together for one central mission, to save the world, or to benefit anyone else. Each joins the crew with an absolute mission of their own, to be the best at one thing or another, or to further their own dream, and the 'captain' wouldn't have it any other way.
The way the crew roams the world, freely, and to experience all the awe and wonder of the world around them.
Although some would think the World Government is an enemy or antagonist in the show, it's really more like they are just in the way, just an afterthought to be bypassed and dismissed by the main characters. They don't subscribe to the ideas of the world government, but aren't all caught up in that denial either, they just keep on doing what they do.
Combine the spirit of One Piece and the ideas in Atlas Shrugged in your mind, and feel unstoppable, a cool feeling.
@Hairnet - There is indeed a character who wields his third sword with his mouth, and he is awesome. At first you're like, in your mouth, really? Soon after though, you develope a deep respect for his style. =)
While I do indeed like many animes, many are about self sacrifice, honor, and duty. One Piece is about Freedom, Honesty and following your Ambitions, which I think makes it Objectivist Gold.