It's adherence to a perfectly consistent and logical power system.
The way the story and nen system compounds on itself in each arc, and long-form over the course of the whole story. Every detail in HunterxHunter serves a purpose in the scene it is introduced, as well as another scene down the line. This multiplicity allows for the convergence of many pieces of setup for truly legendary climaxes.
In other words, it has the best structure of any story I've ever experienced.
HxH is the antithesis of the episodic story, and that's what I value most. On top of having the best structure, it's great at intriguing you with paradox, and communicating all of its ideas with perfect clarity. There isn't a single detail that seems hazy, and you're always completely confident that you know what's going on.
God, I love this thing. Could talk about it for hours.