I'll admit that Donald Maass's The Career Novelist got me thinking about this, but I agree with his sentiments and have stuff to add.
He likes novelists. Not just writers. Well he likes writers, but apparently he loves novelists. He says it has something to do with their intelligence and unique insights.
I've read elsewhere that the sign of a great writer is that he or she will have an excellent BS detector; a well-functioning radar for untruths (Ernest Hemingway is the origin).
I could quote all day what famous people have said about writers, but then, that's already been done, so I'll do new.
There is a magic to what motivates a writer to write, I'll say first. I sincerely worry that if you work too hard to deconstruct the structure around that motive, you could kill the motive. If you look too hard at why you write, you may find something you don't like. The basics for writing motivation, as told popularly, are: expression, a story to tell, recognition, money, fame, emulation (of a respected author) or acceptance.
Your motivation for writing could be because an elementary school teacher told you you were good at it and, Hell, maybe she was just trying to make your day (if you think that's true then you must read the rest of this post, please). Could be a friend or family member that told you you were the greatest writer ever. Could be a lot of stuff. Part of it for me was opening a fiction book and literally thinking, "Are you serious?" I knew immediately that I could do much better than the author I was reading. It's fairly common among writers. But I had the hope-casting, inspirational compliments, too. (See last blog post)
When you cook all of these goodies up in a single pot of psychology (I was an English/psych. double major, writing concentration, but never graduated because I didn't need to), you come up with an interesting cast of characters. Novel writers, or anybody who is willing to write long stories, has to have an interest in lacing words together to create feeling. I'm tired (seriously tired) of the artist analogy--particularly the painter one--that compares the writer to the artist. A writer is not an artist. Words are not art. Communication is art, but not words. Words are not art anymore than a tube of acrylic paint is art. And even after you write them, they are not art. Art is a thing you can look at, right now, and get a feeling. One single (however complex) wave of feeling. It could be ugly, distorted, beautiful, eerie, disgusting, overwhelming, dismissal. Some feeling comes. Words are not the same....
What a story does, it does in stages. It happens with a flow of a song but the length of a day's work. It's not art. It's a long, complex dance. A writer can get this dance onto paper and may not even have a full comprehension of what he has done in the mind of the average reader, and that's where talented, younger writers can thrive. If a younger writer knows the pitfalls and can set his imagination free, he can leap right up onto the levels of the older, more experienced writers but that is a rare exception. The stories he writes must ring true, and because of the limited experience of the young writer, widespread wisdom and truth is difficult to sprinkle across his story. The old, and arguably, antiquated "Write what you know," rule still has a firm place in the thoughts of the successful, younger writer ("It is unbecoming for young men to utter maxims." Aristotle). But even with the young writer, the story is told in stages. It unfolds.
Your reason for writing is not necessarily important. I can predict who will be a great writer by one sign; if a writer tells me that he or she gets a strong adrenaline pump when they write something in their story because they know, not think, but know on the deepest levels that the thing they just wrote will move people and if they like that feeling (some do not, believe me), then that writer, if they choose to make it a career, will succeed.
The first argument against my theory might sound like this; the calm writer that writes something to the tempo of Walden or Huck Finn (I respectfully abbreviate; this is a blog, yo), what, they won't be a success? I didn't say that. All I'm saying is that nothing, but nothing, can stop a writer who experiences a pleasurable elation when he writes--if his heart rate rises and he writes in that half out-of-control state, something special happens. When the desire to get a story out is trumped only by the need to feel that familiar (and it must be familiar rather than rare) adrenaline rush, I would bet large sums of money, if I had them, that that writer will make it.
The things that make a writer tick are not important; all that matters is that the writer tick, and tick hard.