Clarity As you create sentences in your first draft, you will use the first grammatical constructions that come to mind. Once you are revising, it will become necessary to rewrite much of your original work for the sake of clarity. Essays tend to be written one painful sentence at a time, and it is easy to lose sight of the bigger picture.
Do not try to be wordy in the mistaken belief that it will make your essay sound more "serious." See the section on writing sentences, especially the discussion of wordiness and (again) the passive voice.
Coherence. Students often have trouble putting a collection of perfectly good sentences into an order that makes sense. The page on organization talks about the logical processes used to structure an essay; apply the same guidelines to every sentence.
No matter how strong your ideas, if they are disconnected they will have no impact. You are trying to convince, and convincing requires logical, systematic presentation. There are more suggestions on the organization of ideas in the pages on paragraphs.
Unity Check everything you have written to make sure that it contributes to the essay. The strength of your argument will be diluted by irrelevant digressions or redundancies. In the course of writing the first draft, you will probably compose a number of sentences whose only function is to help you think and lead you toward something else. Determine whether or not you need a given sentence to advance your argument. If you are only spinning your wheels, then that sentence must go. Everything in your essay must be there for a reason.