The fascinating world building and the geopolitics. I have a lot of respect for those who build a world from the ground up for their stories.
Since the beginning, One Piece world was built to be very large, complex, and carefully made. The geography itself is already interesting and incredibly well constructed. It feels like a genuine world on its own. The series has continued to add layers to the world. Originally we thought it was just a Pirate vs Navy conflict. Then you find out there are secret organizations, revolutionaries, actual islands and countries with their own kind of culture and warriors.
The issues presented in One Piece world politics are also diverse. Most of them even correspond with real world issue, such as racial discrimination, inequality, corrupt government, civil war, underworld smugglers and slavery.