I would say that the main draw for me was just the positivity displayed by Iñaki Godoy as he plays Monkey D. Luffy. While he is optimistic to the point of naivete, Luffy's good-hearted and loving nature is downright infectious. He sees his world as a better place than it really is and, in doing so, makes his world a better place than it was before. I want him to become the "King of the Pirates" if for no other reason to see him change piracy in his world into a force for good.
The entire cast was wonderful, feeling like manga drawings come to life with anime action pieces. Sure, the plot was a bit juvenile, and there were parts that dragged, but it was still engaging and managed to have some interesting twists. Meanwhile, the world itself was fantastic, in the literal sense of the word. An ocean-dominated world is the perfect location for a fantasy, pirate(ish) adventure.
In short, I'd be happy to watch more seasons of this series.