I wonder how much of their success is due to vala. I assume they're all gifted but vala seemed to be the right vessel to channel their ideas and reach high goals faster.
I would say the greatest strength of Elementary is being very consistent and opinionated, which I imagine the heavy usage of Vala helped with. It really does feel to me like an even more opinionated version of GNOME (heavy usage of Vala and GTK, mandating all apps published to their store also be GTK and open source, having a very specific desktop experience without many customization options).