Frank Herbert on what makes enduring literary art

"Look back on the Artists and Critics whose work endured....

1. They tested limits, their own and yours.
2. They produced something new.
3. They tested you.
4. They entertained you.
5. They often addressed very large questions, sometimes in miniature.
6. They ringed a chord which we still call truth.
7. They took you beyond yourself to something you recognized as better.  They reminded you of the best in your own humanity."

--Frank Herbert, Nebula Winners Fifteen 

[Note: points numbered, mine. Some lines I erased which I felt implied in the above.]