Easily these three traits: creativity, knowledge, and persistence.

Yes, you can copy builds, but you are fated to be meet the same limitations others have approached beforehand. Doing what others had done before. Those who constantly push to newer levels and reach greater heights have tried innovative builds in both item set and skill set that allows them to constantly strive forward. Take a look at Barbarians. When people first played, nobody would have thought to try out this monstrous juggernaut into a healing and utility support for others - but through this conversion, it has allowed a synergy with other uniquely built players to reach towards the upper end of greater rifts. And it is without a doubt that for every successful build, there's countless of other fail builds. But it's not quite failure, it is merely a conduit and inspiration to constantly adapt and be flexible - to try out something slightly different. Perhaps another legendary here, a passive change there, maybe a rune change for that skill - it's all derivation from the failures.

And this lesson doesn't just apply to just D3, it's applicable to everything. Success and being the greatest isn't the people who have paragon XXXX, who stay up all night (or bot), clicking the same buttons and skills repeatedly. That's not impressive, not should it be. True mastery of Diablo isn't some static or arbitrary number you collected, be it speed runs, gold, some stat.

It is the unheard of players who constantly argue with others, with themselves. Asking and being curious as to how a mechanic work. Constantly debating, examining data, and theorizing. To be a visionary and thinker. But to me, being great also means having empathy and passion to share what was learned. To share what they know so others can build upon that foundation.

These are the things that makes a player great to me - and I will probably never know who they are. But it doesn't matter as their contribution has already laid their legacy.