I've been reading rec.arts.sf.composition for ten years now, and over that time I have picked up a fair amount of knowledge about writing and publishing.
Two years ago, I realized that TV had kindly given us an analogy for the slush pile: American Idol.
Your slush pile gets everyone who thinks they can write -- like the open auditions for American Idol. It is clear that most of them are bad, and have no clue as to how bad they are.
Some are truly awful -- those who cannot carry a tune, or in the slush pile, use decent grammar and spelling.
Others are not terrible -- they can stay mostly on key, but don't have much of a singing voice. In the slush pile, they made sure that their manuscripts were spell-checked and their grammar was decent. But the story is not very good.
At the next level, you get competence -- these are the singers who get to go to the next stage of the auditions. These are the writers whose story has some good qualities, but not enough. Someone at this level might get an encouraging rejection, or asked if they have something else... They might get an agent who is willing to work with them at this level, or they might have to wait until the next.
At the next level, the semi-finals, everyone has potential. Here is where the acid test comes. Which, in my opinion is: can this person be a pro? Does this person have a professional attitude, a good work ethic? Can they take criticism and learn from it? Do they keep working to improve, or do they want magic short cuts? If criticism destroys you, you probably don't have what it takes to be a pro. If you take criticism as a personal attack, you are too busy defending yourself to do what is really needed: improve.
In both cases, it is a case of multiple things being needed to excel. As I see it, to make it on American Idol, the singer needs the following things: A good voice. Good singing technique, to keep the good voice. The ability to judge the quality of your performance. The ability to work hard, and the stamina to do so. The ability to take direction, to listen to criticism and learn from it, to decide what is valid and what isn't. To know who you are, and how to keep that distinct voice.
To be a good fiction writer, you have to have interesting characters, who do interesting things [plot], in an interesting place [setting]. It is nice if the writing on the sentence and paragraph level is beautiful, but competence is essential. In order to be a pro, you have to have a professional attitude. Writing is work, and isn't always fun. Your work is a product, and criticism of your work is not a personal attack, but a guide for improvement. You have to know your story well enough to make the changes that will improve it, and _not_ the ones that will take the story the wrong direction.
In any case, final success is a matter of luck, opinion and taste. In the case of American Idol, you have had a LOT of exposure, and may have enough of a fan base to make a decent career even if you don't win. In publishing, it is getting to the editor who likes what you do, and thinks that others will, too.
Well, I hope that made sense. Now to catch up on my newsgroup and lj reading...