The other day I was asked what was the one thing that makes the most difference in writing fiction. If I can choose only one thing, which was the case in that conversation, it would be to learn to recognize the difference between passive and active voice.
The reason that having a friend who is good in grammar or an English teacher edit a manuscript doesn't guarantee a marketable piece of work is that having impeccable grammar isn't enough. While it doesn't hurt for a writer to learn the eight parts of speech, it certainly doesn't make them a fiction writer.
Fiction writers, of any genre, should be able to deliver the basics of character development, story line with arcs, and an engaging beginning with a satisfying ending.
But it is voice, I think, that brings the story from the slush pile to publication. An active voice that shows far more than tells will grab the reader and hold them though to the end. And, if that reader is an acquisition editor of your most desired publisher, all the better.
Good writing isn't one skill; it is a skill set. I've mentioned other skills above that a writer who wants to improve their writing should learn: character development, strong story line, engaging beginning, satisfying ending, basic grammar, showing versus telling (and when to do each), and an active voice. That list isn't complete, but it is a start. Part of a writer's job is to continue to learn their craft, and perfect it.
What tip do you give people about becoming a writer?