In two words? Emergent gameplay.
D:OS has a lot of issues, honestly, but at the end of the day, there are so many different ways to do things, not just in how you build your characters, but in how you approach the battlefield and what happens on it, that those issues can be largely overlooked.
I was never a fan of Divine Divinity, I never got through Beyond Divinity, and I thought Divinity II was pretty damn terrible. Which is probably part of why I initially overlooked Divinity: Original Sin - from the screenshots and so forth, it looks mostly like yet another uninteresting ARPG without any depth, and like I said, not being a fan of Larian beforehand just made me go /ignore.
Furthermore, I find the world of Rivellon to be largely uninteresting, with very few redeeming aspects.
But D:OS is honestly really, really good, in multiple ways. The combat system is probably amongst the best turn-based games I've played, especially considering that it's not built around the idea of squares, but in an open 3D environment.
I hope Larian really takes D:OS to heart, and keeps working on D:OS as a foundation, polishing, expanding, and making it all-round better, until such a point that they have it down to a veritable science, allowing them to focus on world-building and narrative. Because if so, they have an incredibly bright future ahead of them.
