It's interesting that two of the biggest phenom's were also two of the biggest surprises. I think part of the thrill is the unexpected (along with phenomenal natural talent).
How many more are still out there in a myriad of different guises.
Susan also picked a song which seemed to reflect her own life, and which ended up involving the audience in their own pre-judgement of her. So it was more than just the music which made this particular performance such a success. She nailed a whole series of emotional buttons in one shot. Very powerful stuff.