When I teach writing improvement to professionals, I often start out asking them to describe the bad writing that comes across their desks. Here is the list we generally create:
Missing words and double words
Never gets to the point
Long, complicated sentences that don't really say anything
The first three are the result of either poor education or more likely poor or no proofreading by the writer. Most native speakers do not make grammatical errors in their speech (if they do, it's occasional and always the same ones) but we make them when we type quickly and don't check our work.
The last two are more problematic. I'm currently editing a Master's portfolio in psychology and counseling and it's awful. If you just glanced over it, it would look pretty good. He uses big words that seem important and academic and there are no glaring errors--at first glance. But lots of the sentences mean nothing. There is no grammatical subject + verb + object, just strings of phrases that he has learned from the textbooks he's read or lectures he's attended. The sentences have a convoluted nature to them that doesn't replicate normal speech patterns; again it's just phrases strung together. In addition, he has used many of the same sentences over and over, verbatim--one of the hazards of cut-and-paste writing.
He's going to be very unhappy when he gets this back from me. First, he thinks he's done and that I'm polishing up the occasional error. Instead, I've included at least three comments per page on 120 pages for him to deal with. Second, from the resume included in his portfolio, I see that he has always gotten good grades (A- average) both as an undergraduate and as a graduate student. My guess is that his teachers have been flummoxed by the writing and just let it go since they didn't know how to help him. He's not going to take kindly to my eagle eye even though he has paid for this.
He told me in an email that he wants his portfolio to be exceptional. I hope he meant it!