1. The humility to learn. There is a myth that good writers are just born that way, or have natural flair or talent. I do not believe this is always the case. In fact, from the bits and pieces I've read about authors, I suggest that some of the best are those who have failed and failed and failed, who knew they were junk, and who grew into their role with both a hardness and humility that is enviable. Let me put it straight: writers receive far too much rejection and criticism to be proud. It is vital that you believe your actual writing to have no merit. The idea must carry you.
2. The determination to persist. Just as people are not born natural orators, they are not born natural writers. A child must learn to talk, and a child must also learn to write. They start with their alphabet. At age ten, they may have some hackneyed phrases that they use to put together a narrative. At twenty, most will have essay writing skills and a thorough fear of penning things fictional. Again, I say that writers receive too much rejection and criticism to be wimps. If you want to succeed, you are going to have to crash and burn. It's not like learning to ride a bicycle, and falling off, and trying again. It's more like flying a rocket ship, and blowing up several times, being reincarnated, and trying again (and risking appearing to be maniacally suicidal).
3. The ability to recycle well. Nobody is original, but writers - the good ones - will be resourceful in how they use ideas. They will not reuse them in a cheesy sense, but choose a different angle or a different light to portray things. Remember that the same rose looks different when it is half in shadow.
4. The ability to be human. A good writer is never just a writer. Writers are good when they enlighten us, when they show us what it is like to be human and they illuminate our experiences. Writers demonstrate the meaning in the mundane, the truth in the mess. A good writer is not one that sits at a laptop all day. A good writer might be teacher, comedian, check out operator, taxi driver, or lawyer. The more human interaction, the better.
5. The ability to be ruthless. Ever had the scene or sentence that you just loved? It made you look sophisticated, intelligent, and elegant. Get rid of it. If you love it, often you don't need it. If you can't delete it, it doesn't belong. It isn't serving the book; it is serving you. Always remember that you don't write for yourself. Words are meaningless on a desert island that only one person inhabits. You were made to communicate. Be ruthless with your ego and serve the people.