I believe the answer is nothing. Except the burning passion to write and continue to write no matter what. And that we constantly seek to improve yourselves as writing can never be a craft which someone is perfect at. Mastering it is in due time, but perfection is impossible.
#1 All Of Us Have Different Dreams Even As A writer
Some dream of writing a literary masterpiece acclaimed by the world, others want to simply tell the best stories for the characters they love. Some simply want to be famous or want their story heard as far as possible. This are all different dreams.
Hence, it is pointless to assume that writers should have the same benchmark to mark them against. As we all have different wishes and dreams and especially in writing it will be different. So, I believe that almost nothing defines a good writer, except that they are driven, working hard and constantly seeking improvement.
#2 Writers need not be good at all sorts of things
Some writers jump genres even when it is not encouraged. Many write in the same genre over and over, building their name. We know Stephen King is the King of Horror, Agatha Christine became famous due to her mystery novels which is best-selling even till today.
There is almost nothing which says that a writer must be good at one aspect to be famous. Some write novels better, others are skilled at short stories. While some heavily prefer micro fiction or flash fiction. And some like me realize their ideas to be mostly serials.
Hence, it is difficult to say that a writer must be good at a particular aspect to be considered a good writer. Some are better at longer works, others are better at shorter works.
#3 We All Have A Unique Style
We all have a different style and voice when we are a writer and we are told to hone it. It is what makes us stand out amongst all of the others. And that it is how we leave our mark. Ideas are easily replicated as nothing ever has been so, what we do is either use our thoughts of the same premise, the same setting. Or how we use words to express ourselves.
And as such, some writers do better with longer words. Making it seem as though it has presence, it makes sense. Some writers are straight to the point and concise. So, you can say that there really is no benchmark as to what defines is a good writer. Other than passion of course.
So, I believe nothing defines a good writer apart from the fact that they are passionate and have the determination to struggle. What do you think makes a good writer? I will like to hear from you below?