This guy boils it down to 6 writing tips here.
Read the whole article for the explanations, but the tips themselves are:
1. Kids like chaos… but only a bit of it.
2. Kids like adults to be larger than life… but not all the time.
3. Kids like to be independent… but have friends close by.
4. Kids like to be scared… but only a bit.
5. Kids like new games… and old ones.
6. Kids like new ideas… and old ones.
It seems to boil down to “Hit the right market at the right time” – and if you don’t you can write something that follows every rule ever stated and it still won’t fly. The point here is that Rowling wasn’t writing to these rules when she was writing the Potterverse books. These rules were derived FROM the wildly successful Potterverse books after the inescapable fact of their incredible success. But before there was that… there was a writer, writing a story that she wanted to tell.
That’s the first rule of success, really. Write the story you want to tell, not the story that someone else thinks that you should be telling two years from now when the trends change. If you write what you love and you have the luck of hitting that market in the right place and at the right time, then people will be analysing YOUR books for the rules of success.
Even though you never wrote by numbers to begin with.
Now go (those of you who are reading this who are writers) and hug your plot bunnies, and write the thing you love. SERIOUSLY.