His modesty and genuine love for basketball. According to this interview, he self proclaimed that he feared he would drop to the second round in the draft.
Skill or not, there's an added bonus to any player's potential when they are humble about their game and experience and genuinely love basketball.
That's why guys like Russell Wesbtrook and Joakim Noah thrive so much. They have passion for the game and want to play. And why guys like Bynum never touch the court again - no passion.
Embiid is going to be a guy that conditions himself enough and takes rehab seriously enough that he'll be fine.