Who's a good writer?

Here's a post that's probably been pontificated on at least a million times.
What makes a good writer?
Ok, you have to be able to formulate a good sentence. You have to be decent with grammer and you should be able to spell a little.
It's more than that though, right? I know plent of folks who can formulate the perfect sentence, grammatically, but that doesn't make them a storyteller. I guess it should also be noted that many of my favorite storyteller's play pretty fast and loose with the grammatical rules anyway.
So what is it?
Here's some background. Got another rejection today, this one from an agent. Rejection letters are something that happen pretty regulary. I write to please myself. Tim (Green) sees it the same way. Not looking to impress anyone too much, however, we love some of these characters and stories and would like to share them. It's not about money, fame, celebrity or any of that. We want to share our stories with like-minded people and maybe entertain them a bit in the process.
So we write, we publish and sometimes we submit. Why not try some traditional avenues as well, right?
Sometimes we get accepted and sometimes we get rejected. This happens to the best of them. Tim Burton gets ideas rejected all the time. So it shouldn't bother me...right?
Well, it does bother me. I don't know why. I go through stages from "to hell with you, . You wouldn't know a good story if it crawled up your pants leg and bit your dangling participle!" al the way to "We are the suckiest writers ever and should just go bag groceries for a living so our lack of creative energy won't poison the rest of society." And finally, I come back to "Soldier on, Bastards" (to borrow a phrase from Paddy and The Rats).
So what makes a good writer? Can it be "ensured" that you will be good? Lots of folks come out of programs granting a Masters in Creative Writing. Are all of those people then Masters? Do they get published to wide acclaim (I don't think so, but I have no data). Here are some facts on my favorites authors.
I know Stephen King has a degree in English. JK Rowling majored in French and The Classics. Joe Abercrombie studied Psychology. Terry Brooks majored in English Literature and has an advanced degree in Law. George RR Martin - degree in journalism. Robert Jordan had a degree in Physics.
J.R.R. Tolkien was, primarily a linguist. Robert Howard was basically a well-read, self taught prolific writer who piled up rejection slips for years before becoming successul. Fritz Leiber studied philosophy, but was a Shakespearan actor from the time he was a child, thanks to his parent's acting troupe.
Brian Jacques attended the school of Hard Knocks basically. No formal training as a writer, but lots of life experiences to draw from. R.A. Salvatore majored in Communications and English. William King, while his bio is vague, seemed to gain his writing expertise through work as a game designer. Nathan Long seems to have developed by trial and error, becoming first a screen writer before moving into novel. Ed Greenwood is a librarian who has been writing stories since he was a child. D&D gave him the outlet for a world he had conceived years earlier. D.A. Adams has a Masters in Writing but will be the first person to tell you that he thinks it is not necessary for writers. Mary Robinette Kowal (my cousin by the way...I'm a shameless namedropper) is a legendary puppeteer (like works for Jim Henson productions) and a voice actor (actress?) who also happens to be a kick ass writer.
So what do these folks have in common? Creativity, sure. Great storytelling ability, obviously. But what makes one better than the other and is there a path one could set out on, today, to end up at the same place as this list of folks. Some richer than others (King, Rowling), some with more work under their belt (Jacques) but all prolific, well-loved storytellers with books deemed "acceptable".
Not all are flawless at grammar. Not all have the credentials of a great education in writing.
I suspect they just had a story to tell, so they wrote it. Then they submitted it, again, and again and again. In some cases, it might have been accepted on the first try and gotten a huge advance. In other cases, it might have taken so long it seemed like a fools errand. But they kept going. Of course there are persistent people who feel like success continues to elude them.
So what's they key? I don't know. I don't have it. I suspect a lot of these folks don't know what it is either. I am sure in many cases they know what it isn't.
Puzzling. Felt cathartic to write about though.