1. When an author has forgotten who the audience is. It is not so much that technical language should be avoided (or long sentences, or complicated strings of words), but that it should be avoided when writing for certain audiences. So what should be first on our minds when writing is our audience. The language and sentence structure should support that. This doesn't mean that we should over-complicate things though. I must admit that I dislike reading most philosophical writers because I hate to muddle through the language.
2. When an author has misappropriated quotations, misunderstood a person's work, or put words in a person's mouth that are not that person's words or thoughts at all. We should be very concerned about fair use and honest representation of another person's work. It is my personal opinion that we all must be more careful when we write that so-and-so thinks/believes/says something. Does that person really think/believe/say this, or are we as authors creating a position for another person that is not really his or her own, in order to push our own agenda?
3. When an author casts his or her criticism as polemic. If you won't say something face to face to a person, or in a public venue with the person sitting there, you shouldn't say it in writing. Mark Goodacre's advice is seconded here.
4. When I finish reading an article or a book chapter and I have to stop and figure out what it was about or why what I read was important. It is up to the author to tell the reader this very clearly, in the opening and closing of the piece, and often in between. I always ask my students, undergraduate and graduate, to boldface their theses. Why? So that they make sure that they have one!
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