In terms of the Kava, literally just the place they're grown, but that has a lot of implications...
In terms of business, Hawaiian farmers have more control over the consistency and type of Kava they are growing. They also don't have to face high shipping costs, which means their Kava should be much cheaper than imported Kava. In essence, it should have a good and consistent quality and a lower press than foreign competitors.
Tongan Kava, to me, is pretty consistently the best in terms of flavor, effects, and freshness.
Fijian Kava is sometimes tainted with cheaper Vanuatuan Kava, such as imported wild Kavas from Vanuatu or PNG. That being said, all of the vendors I have reviewed sell great Fijian Kava! A lot of crappy "Fijian" Kava at Kava bars is mixed with terrible Kava. Tonga doesn't seem to face this problem.
Vanuatuan Kava is pretty consistently the best. It is the heartland of Kava. They have the strongest varieties of Kava, and also the widest variety. Unfortunately, Vanuatu is a pretty wild place and consistency can be an issue.
The botany of Kava is a separate topic. Kava migrated around the Pacific for thousands of years and cultivars evolved and had different traits selected for in that evolution and diffusion.