Shweta said:
"Great storytelling means having a great story" is surely circular?
Depends on how you view the question. I see two potential questions here:
1. What events catch our interest so that we want to hear about them (great story)?
2. Why does Grandpa bore me to tears while I could listen to grandma forever, even when they're telling the same story (great story teller)?
As to Rowling, I don't think her story's too special, but she's telling it well. Her main strategy, I think, is that she's writing to the cliché and just when you're comfortable with your judgement of a character she pulls the carpet from under your feet. She could have been a bit more consistent, there, I feel. For example, if you view the houses for values you end up with Slytherine = bad, Griffindor = good, and the other two are insignificant. I also think the all-power of (motherly) love gets a bit corny eventually, but since that's the thematic and ideological core of the books, I don't really mind that as much as the house-thing. Still, she keeps up enough of an ironic distance towards her stereotypes that she managed to hold my interest.
In the end, much of it is subjective, so that you'd end up with a typology of readers along the dimensions of "what events interest me?" and "what story tellers manage to seduce me?" If people are looking for a one-size-fits-all-approach I suspect they'll end up with wide appeal, but little intensity.