To me, it is that the Pebble simply works, and does it well. I've tried wearing an Android-based smartwatch - while great hardware (except battery time) - it simply didn't work that well. I think that is the biggest selling point.
On the other hand, I remember having my Pebble in a drawer for most of the first year. But once SDK 2 was released, things started to happen and it became really useful in itself and the apps/watchfaces available.