Here is an outline of what readers look for, and what writers should strive to accomplish, in effective writing. 

CORRECTNESS refers to proper grammar, spelling, and punctuation — observance of established conventions.  Here we are concerned with clear-cut matters of right and wrong.  These are sometimes referred to as the "basics" or the "fundamentals."  Thus, correctness involves not just grammatical rules but also proper punctuation and the use of correct words. 

CLARITY refers mainly to sentences, although, since sentences are made up of words, diction (word choice) is important.  Diction affects clarity because imprecise words make the meaning unclear.  The main concern is:  Does each sentence communicate the desired point, without confusion?  One key to clear sentences is effective syntax (sentence structure):  the order of and relationship between words, phrases, and clauses in a sentence. 

CONCISENESS refers to the economical use of words.  Concise writing involves the elimination of:  unnecessary words (redundancy/"deadwood"), roundabout expressions (wordiness), and needless repetition.  The objective is directness — expressing ideas in the most forthright way. 

COHERENCE refers to the logical flow of ideas.  Since flow (or continuity) is important for the sense of the writing (its understandability), coherence is related to clarity.  Flow is also interwoven with the organization of ideas; thus, coherence is related to control.  An important consideration in developing coherence is transition — logical movement from one idea to the next. 

CONTROL refers mainly to total organization but is also a key element in correctness, clarity, conciseness, and coherence.  Unity is an important element here.  At the paragraph level, unity  means that each paragraph develops a single, controlling idea; each point in the paragraph clearly relates to that idea.  On a larger scale, unity refers to the organization of the total work; all paragraphs relate to the purpose (objective) of the work. 

To an extent, these characteristics overlap.  For instance, sentences that are incorrectly punctuated will lack clarity, paragraphs that contain repetition are usually poorly organized (and lacking unity), and writing that is wordy suffers from lack of control.  To communicate effectively, any piece of expository writing must exhibit all five of these qualities.

A sixth "C" — CONTENT — is essential.  If what is written lacks substance, it is worthless, regardless of its rhetorical excellence.