If I were to identify what I think is the most frequent mistake of the writers of bad writing we receive, I would say that it is the same that most of us have with speaking: we express ourselves without first thinking clearly and carefully—and with brutal self-criticism--about what we are going to say and how we are going to say it. We have the impression that our minds are pregnant with valuable thoughts that really must be heard without recalling that unless exercise the labor and discipline necessary to put them in a form that can be understood by others, as thoughts-to-be-shared they remain sterile. This is why writing is an art--a rhetorical art--and a process that, while it depends on good thinking, should be thought of as another thing entirely. For while good thinking depends on the internal coherence of the thought itself, good writing, like good speaking, is the art of word-use and knowing the mind of one’s audience.
Read the whole thing--especially if you are a student or an aspiring writer!