(after reading Looking for Alaska and The Fault in Our Stars)
1. His characters are smart. They use a big vocabulary and
2. They talk about big ideas. His books are centered on Big Questions like The Meaning of Life and Death
3. His teenage characters, in the midst of talking about these Big Questions, are still real-life teenagers enough that they just might throw in a your-mama joke. (Or the maturity equivalent). They feel real, and his timing is spot-on. In other words,
4. He's funny.
5. Unlike most YA fiction, in which parents are either dead or criminally negligent or criminally clueless, the main characters in both books having Loving Parents. Whom they like. The parents are just capable of giving the protagonists space to have their own stories.
6. This Author's Note in The Fault in Our Stars:
"...This book is a work of fiction. I made it up.
Neither novels or their readers benefit from attempts to divine whether any facts hide inside a story. Such efforts attack the very idea that made-up stories can matter, which is sort of the foundational assumption of our species."