How important are writing style, grammar and spelling for the Analytical Writing essays?
They, of course, factor into your score, but the graders are looking more for clarity of construction and thought than for Shakespearean prose. The biggest mistake people make on the essays is relying on their strengths as a writer without paying attention to the particular tasks outlined for Argument and Issue essay questions. If you digress from either the topic or the tasks for the essay types, you will not score well no matter how well you write.