what is a good writing

Good writing expresses a clear point, is tightly structured, grammatically and syntactically correct, substantive, and interesting.

To express a clear pointmeans to convey the writer's main idea or--in the case of descriptive writing--the significance of the object, place or person described; in other words, an attentive reader should be able to grasp the writer's purpose.
To be tightly structured,writing should contain logical or associative connections and transitions which clearly express the relationship of the ideas described.
To be grammatically and syntactically correct,writing should adhere to the rules of Standard American English, including proper punctuation and spelling. If writers choose to use unconventional syntax, they should be able to justify their choices.
To be substantive,writing should convey the impression that the writer is informed about the subject. The writer need not be an authority on the subject but should demonstrate awareness of its significance and its implications within a specified context. Informed writing might include any or all of the following: citations of authorities, experiential evidence, discussion of debatable issues related to it, and relevant questions it raises.
To be interesting,writing should engage its readers through original insights and precise, uncliched language expressed in a "human" voice. It should demonstrate the writer's awareness of the specific audience for whom she or he is writing (the audience's degree of knowledge of the subject as well as its age, ethnic background, gender, and assumptions).