The goal actually is to be coherent, concise and compelling while answering the question asked with specific and concrete examples from your background.
Common mistakes
1) not analyzing and breaking down the question to understand it fully and seeing the difference with similarly worded questions
2) not answering the question asked
3) answering with “lofty” prose and not specific and concrete examples from your record
4) not having coherence as in a “theme” for the paragraph or essay. This often is applicants not noting the concept or characteristics they are trying to emphasis from their activities or background
5) unclear writing. Confusing sentence structure, too many descriptors, just plain lousy. The shorter the essay, the tighter the sentence structure needs to be. You need to edit sentence by sentence:
6) poor editing and proof reading. It takes about 4-6 drafts/proofs to have a highly polished product.