What is good writing?

If we want to teach writing to our students we have to have standards
 about what is good writing.
How do we judge good writing?
There are three categories of writing that the teacher and the student
 have to be aware of.
The first is content.
The content has to be interesting, the writing has to have a message.
 If there is no message in the writing, whether it be an e-mail, an essay
or a story, there is no reason to write.
The content has to be coherent.  The ideas must be developed and clear.  
The writing has to be relevant to the topic.  If the assignment is to write a
 restaurant review, then don't give a recipe.   The writing has to be clear.
Your readers need to be able to follow your train of thought.

The second category is technique or skills of writing.  These skills include
 spelling, punctuation, syntax (word order) vocabulary, grammar and sentence structure.
 English is not forgiving if a writer doesn't follow these technical rules.

The last category is form.  This category is also concerned with rules.
This could also be called the page layout. How does the page look to the reader?
 The layout is different for different kinds of writing.  If you are  writing a
formal letter, you have to use the formal letter layout.  A newspaper article
 has to conform to the newspaper space, an essay has to have paragraphing and a title.

Nowadays, more students hand in essay that are typewritten, but here a note
 about font choice is necessary. Choose a font that is readable, and not too fancy.
 You want the reader (maybe your teacher) to focus on your ideas and not be
distracted by a fancy font.

If you do hand write you writing for the teacher to evaluate, then make sure
 that your handwriting is neat and easy to read.  Many times I had to spend
too much time trying to figure out a students' handwriting.

If you pay attention to these three broad categories, you will become a good writer.