When I was thinking about what makes someone interesting, I started coming up with a list, like knowing languages, playing instruments, traveling, but then I realized I was making a list of what I value in my own life. Doing one thing really well doesn't foster a great deal of interestingness. For example, Paco de Lucía, I heard an interview with him. He was not dull, but he mainly played the guitar in his room from the age of 9 or 10. Didn't go to school after that. It is a tremendous depth but a narrow channel. That's how tennis players are.
I am not particularly interesting though, because I lack one key ingredient: varied life experience.
I do think I have an interesting mind, but once again, I am am biased because I am interested in what I am interested in.
From my own perspective, I find a lack of intellectual curiosity in some of my earlier development. Maybe it's that narrowness of youth.