For me the definition of a good writer is someone whose prose you must read, who makes writing look easy (because the reading’s easy) and whose work you look forward to. Good writers show brilliant characterisation, plots that work (within the world they have set up, not necessarily this one!) and they use economy of language – i.e. every word counts and you couldn’t imagine editing anything. Sometimes they add to the language but you get the impression they love English and enjoy “playing” with it.
I know I’m a huge fan of wordplay and puns (as was Spike Milligan). A good writer produces work regularly and edits their work thoroughly too. I need to get the feeling there was no word they could have taken out to make their stories any better. A good writer is committed to their stories, wants to talk about them and knows their characters inside and out. A good writer loathes the idea of celebrities claiming to write when we all know they use ghost writers. At least be honest about using a ghost!
A good writer will find time to write, no matter how busy they are, and will use this time regularly, knowing work will build up. A good writer will write regardless of publication. They know they need to get the stories in their head on to paper. Good writers aren’t afraid of editing either. They also know it is okay to write rubbish, indeed expect to do so but then edit ruthlessly to get at the real story, the good stuff beneath. A good writer does not give up.