(1) Don't make claims that you cannot support. Beware of "totalizing" words such as "all," "always," "never," "every," and so on. Even if you can't think of an exception, your readers often can--or they will be distracted while they try to come up with exceptions.
(2) Don't talk down to the reader. Although in a business setting you might know more about your topic than your reader knows, in a classroom setting this is likely only if you have done significant extra research. Although a student essay normally should read as though the writer knows more than the reader, remember that this is just a healthy convention to help you learn to write better. Talking down often takes the form of an unnecessary definition, frequent repetition, or proving the obvious.
(3) Don't distract the reader from the actual essay. Distractions include the following: lots of exclamation points, more than just a few words in italics, words in boldface other than essay titles and section headings, unusual margins, titles or section headings in special fonts or greater than 14 points high, any stretching or compression of the text to fit a page minimum or maximum, and so on. Once in a while you can get away with an especially apt picture such as an editorial cartoon or a panel from a comic strip, but don't overdo it.
(4) Don't over-generalize, especially in the first sentence or paragraph of the essay. "Funnel" essays (see above) are most often guilty of this problem. You might be startled to learn how many essays begin like this: "Every society has people in it. One of those societies, England, has millions of people. One of the most populated cities in England, London, has a great diversity of people. This essay will describe the main ethnic groups of London." Readers are put off by openings with bland generalities. Likewise, they are put off by openings that provide a common proverb or a dictionary definition of a term.
(5) Don't write extremely long paragraphs. Readers tend to get annoyed at the prospect of having to wade through a big block of text. A paragraph that goes on for two thirds of a page looks daunting. Aim for eight sentences per paragraph as a reasonable maximum. And remember: one main point per paragraph, please.