This might not be a great contribution, but perhaps we could look at the writers who have written predominantly YA and children's books and afterwards tried to cross over to adult fiction (more or less successfully).
I would say that J. K. Rowling has managed to keep her writing style throughout The Casual Vacancy, but the story didn't appeal to me at all. On the other hand, Gaiman is struggling to find balance between the perky children's book style of Anansi Boys and the dark, gloomy atmosphere of American Gods. Both good writers, but this search for the answer to the same question you asked is visible.
I don't like how YA is mostly formulaic and I really don't have the patience to read it anymore (so many dystopian situations that cause our young characters to rebel against the existitng system and then they- fall in love). However... the fact that The Hunger Games had a catalogue of characters that were super interesing made it more accessible. So, we're back to the characters, aren't we? I think the forumers are pretty much unanimous on this.