Certainly, it is not grammar or punctuation. It is quesionable that you must even learn these skills. As an example, I offer e.e. cummings.

What I do think makes a good writer even more than imagination, although it is a part of imagaination, is the ability to make the reader experience what it is you see in your head. Is the reader there? What is the reader thinking while they are reading. Are they so wrapped up in the characters that they dont see you have misspelled words, or not used quotation marks? If they can put the story down and say, "Wow, I liked that. I wish it hadnt ended." Then you may be called a writer, regardless of technique, style, imagination, or punctuation. The abiiity to make yourself understood by someone elses. To grok someone or to make them grok you.

IMHO