You could ask fifty different writers and get fifty different answers. Yet we all know bad writing when we see it. Bad writing evokes zero emotion. It fails to connect ideas. Bad writing may tell a story or make a statement eloquently, but it fails to make that narrative or point something a reader can relate to.
That foreshadows my answer to what makes a good writer good. I believe a good writer uses their skill to connect a reader to the story (or point of the essay, or meaning of a poem, etc.).
Good writers build their writing in a manner that can be understood. They write in their own unique voice - not mimicking someone else, or saying things that seem stylistically acceptable to stodgy intellectuals. And good writers don't always play it safe...they take risks to make that connection with their target audience, even if it means saying or doing something not politically correct or trendy.
So what do you think makes a good writer good?