Having recently finished some editing projects, I have come across the following trends:
1. Flabby writing
2. Overuse of passive voice
3. Failure to properly cite quoted material.
With regard to flabby writing, most people tend to write in a conversational tone. Verbal conversation doesn't necessarily translate well to written narrative. One pads conversation with filler words that quickly weigh down the written narrative. Writers also season their content with too many adverbs.
Overuse of adverbs lends itself to purple prose, overly descriptive, effusive, flowery writing, which has generated a school or philosophy of writers who make it a point to strike almost adverb from their (and others') writing. I happen to think that adverbs enrich the language if used in moderation. Adverbs, the complaint goes, tells how something was done. "Showing" is most often better than "telling;" however, if the event just isn't all that important, then "telling" effectively gets the point across without wasting too many words.
Powerful when used sparingly, passive voice oftentimes serves as the default for writers who haven't yet learned that active voice packs more punch. Pairing a verb with any form of "to be" (am, is, are, was, were, be, being, been) drags out the action and slows it down. Simple past tense has greater impact.
Don't get me wrong: I'm not against passive voice. I'm against overuse of passive voice. Quite frankly, reading something written almost entirely in passive voice bores readers.
Many writers use quoted material. I'm one of them. However, one should not quote (or misquote, in some instances) material that one does not properly attribute. Whether the material has long since passed into the public domain (e.g. biblical quotations) or is the lyrics from a recent song, proper attribution prevents accusations of plagiarism. Readers quickly lose respect for authors who plagiarize and copyright holders can rightly sue for damages. The real concern, I've noticed, comes into play with business writing. Proposals, reports, blogs, and other content may be copied wholesale and used for business purposes. Doing so has no more moral or legal right than not citing quoted material in fiction writing.
Regardless of frequency, I do believe that writers and companies which fall prey to that temptation do not mean to steal--plagiarism is stealing--but they simply get lazy and find themselves butting up against deadlines. Better time management would help prevent that.
Editors serve a useful function in tightening flabby writing, strengthening weak prose, and catching quoted material for proper referencing. If you think your writing is darned near perfect, then hand it to an editor. You may be surprised. I always am when I do the same.