The primary purpose of writing is to communicate. If you fail to communicate your concepts, arguments, mood, ideas, characters, or scenes to your readers, you have failed the primary task of the writer.
The problem that I encounter most often is the attempt to impress readers by using big words. This shortcoming is common in business writing and academic writing, but from these incubators the infection has spread far. As William Strunk, Jr. writes in The Elements of Style, "avoid the elaborate, the pretentious, the coy, and the cute. Do not be tempted by a twenty-dollar word when there is a ten-center handy, ready, and able."
Use adjectives as adjectives, nouns as nouns, and verbs as verbs. A complex argument is better than a complicated one. A thesaurus and a dictionary should be your constant companions when you write.
Avoid buzzwords, jargon, and clichés. They are ripe with ambiguity. You are not leveraging your prose with lingo. If you are results-oriented, get your ducks in a row, and remove the low-hanging fruit of buzzwords, jargon, and clichés from your writing. At the end of the day, you will write clearly.
The other problem that I encounter often is the use of complex sentence structures. Sometimes, this shortcoming derives from an author's desire to appear erudite. Other times, the author has not developed fully the idea that he or she wants to convey. When our writing becomes too complex, we should ask ourselves if our thinking is clear on the subject. Do we know what we want to say? Whatever the origin of complexity, the result is often unintelligible. Keep your sentences and clauses as short as possible.
Trying to force a sentence or clause into passive voice is the most common source of verbiage. Write in active voice whenever possible. Like your boss, your readers do not want to hear that mistakes were made. They want to know who did what to whom. Have subjects act on objects.
Try to keep your subjects close to your verbs when composing a sentence. Place as few prepositional phrases, subordinate clauses, and non-restrictive clauses between the noun and the verb as possible. Place the noun and the verb close to the beginning of the sentence as often as possible.
Avoid redundant clauses and phrases. When we repeat ourselves while writing, the reason is often because what we are not certain what we want to convey. The process of writing can clarify your thinking on a subject. Never be satisfied with your first draft, because writing that first draft may have clarified what you think about a subject or how you imagine a character or scene.
Use adverbs sparingly. Oftentimes adverbs are redundant. She should not enter a dimly lit room. She should enter a dim room. Is there a different adjective or verb available that conveys your thought than the adjective that you want to modify with an adverb? Why would she run quickly if she can sprint?