Writing involves you in producing a product. The product is called a text. Now texts can be approached and understood differently via any of the nodes on the author/audience/message triangle. Like the whole author/audience/message triangle and the rhetorical situation it models, texts exist in culture, community, and society. Culture, community, and society are subtly different ways of thinking about groups.
Communities are groups which come together for various purposes. Those of us who study writing spend a lot of time talking about discourse communities. At the simplest level, discourse communities are groups of folks who share a discourse or conversation. The discourse can be oral, written, printed, digitized, etc. Folks may or may not meet in order to share a discourse. Those of us in this class constitute a discourse community. We share guidelines to make talking to one another easier, things like this blog, email, or the Q&A section of the class blackboard site. We share expectations about the subject matter and content of our discourse. Think, for instance, how odd it would be if I suddenly started sending you posts about, say, sailing. We share expectations about what constitutes a reasonable investment of time in this discourse. Would you, for instance, accept ten emails about writing a day, all on different aspects of writing and requiring, in sum, several hours to process? There are discourses, like novels, newspapers, trade journals, RSS feeds, etc., where such an investment of time and energy pay back and are expected. You get the idea.
A discourse community is a group which comes together to hold a conversation and shares expectations about matters such as content, format, kinds and numbers of communications, ideas of evidence, a power structure among its members, genres in which communication will happen, etc. et etc.
Discourse communities are often governed by the attitudes of the societies in which they are embedded. Societies are groups of communities. These various communities share to some degree experiences, knowledge, rituals, technologies (ritualized ways of doing things), tools, and ways of looking at and understanding the world. For example, consider the American society. We share a government, a technological infrastructure, many (but not the same) attitudes, etc.
Now, think for a moment about how our discouse community is embedded in and governed by our society. We come together within an institution--the college--which reflects and is governed by many of the attitudes of society. Education as a means of obtaining and improving one's social status is a good example, but others would include how this class is structured, the basic notion of student and teacher, our roles...I could go one, but one last word about society: many times, when you talk about culture--the American culture, for instance--what you're really talking about is society. Culture is best thought of as the world view, knowledge base, and practices shared by a community or society. Cultures can interpenatrate different societies, but a society would be impossible to construct without, at some level, a shared base of cultural practice.
OK, here's the breakdown so far: communication is limited by the accepted practices of the society in which it takes place. Most communication happens among specific subgroups within a society called discourse communities, and discourse communities develop a set of shared expectations and practices--that is, a culture specific to that discourse community. Write if you have questions, as you just got thrown an overview of the basic terms in the sociology of communication.
What does all this have to do with good writing? For a hint, take a moment and read the following quote: "Often the accurate answer to a usage question begins, 'It depends.' And what it depends on most often is where you are, who you are, who your listeners or readers are, and what your purpose in speaking or writing is" (Kenneth G. Wilson, usage writer).
The reason why professors shouldn't offer easy answers for what constitutes good writing is these answers are specific to the rules of discourse and society in which the writing is embedded. What constitutes good writing depends on the culture of the discourse community in which the writing takes place, and--most importantly--it depends on the rhetorical situation in which the writing takes place.
Good writing is, hence, a relative term. What is good writing for the author may not be
good writing to their audience. Good writing in one discourse community may not be good writing in another. Good writing in one society usually isn't good writing in another.
Keeping all of the above in mind, my own working definition of what constitutes good writing is defined from the author's point of view on the rhetorical triangle. That is, good writing is that which effectively accomplishes the author's intended goals. Notice this definition still is relative to the rhetorical situation, discourse community, and social expectations in which it takes place, but I can imagine a rhetorical situation where I wasn't a member of my audience's discourse community and didn't share the culture or society of the person with whom I want to communicate and where I am still able to achieve my intended goals.
From the author's perspective, all that matters are intentions and goals, that is, your purpose. This is another reason we have the notion of the discourse community. It makes our lives as author's easier. It's easier to learn the practices and assumptions of the discourse communities of which we are a member than it is to learn the practices and assumptions of every person with whom we want to communicate. If we have a good idea of the expectations of the discourse community with whom we want to communicate, we can cubbyhole this a whole set of readers. Because people are endlessly variable, we won't understand well enough to know exactly how each member will interpret us, but, by-in-large, we will know well enough to do what we intend with the audiences as a whole.