I talk sometimes about power slots -- those sentence positions with the most potential for impact upon the reader. The main verb, the subject, and the direct object are the main power slots (with the verb being most important), and others include indirect objects, objects of prepositions, and maybe well selected adjective and adverbs.
So, Gary Lutz wrote an article some time ago in The Believer, and I just got around to reading it this weekend. He talks about power slots, and I thought it would be worth quoting that here.
Before we turn our eyes and ears to the entirety of a two-clause structure by Christine Schutt, maybe we can agree that almost every word in a sentence can be categorized as either a content word or a functional word. The content words comprise the nouns, adjectives, adverbs, and most verbs: they are carriers of information and suppliers of sensory evidence. The functional words are the prepositions, the conjunctions, the articles, the to of an infinitive, and such—the kinds of words necessary to hold the content words in place on the page, to absorb them into the syntax. The functional words in fact tend to recede into the sentence structure; their visibility and audibility are limited. It’s the content words that impress themselves upon the eye and the ear, so the writer’s attention to sound and shape has to be lavished on the exposed words. They stand out in relief. (Pronouns, of course, do not quite fit tidily into this binary system; pronouns tend to be prominent when they are functioning as subjects or objects and tend to be shrinking when they are in a possessive capacity. And some common verbs—especially those formed from the infinitives to be and to have—tend toward the unnoticeability of operational words.)
The entire article is worth a read. It's long, but the biographical material is intriguing and his discussion of sentences is fascinating.