Written English
Probably the biggest sin of all, for all less than excellent writers of English, is their grasp of punctuation; and the biggest failing there is the use of a comma where, structurally, only a semicolon will do.
This is not controversial; it is not difficult; I own a dozen books on punctuation and all the major works on usage and style, and they all agree on it. It is also very simple. You use a comma before "and" or "but", or to mark off a qualifying phrase; however, you do not use it to separate main clauses without any accompanying conjunction.
Apart from the above, called the "comma splice", most punctuation problems are about sentence structure, in other words the deepest fundamentals of prose composition; and those without a good grasp of this will only ever be guessing about where to put punctuation marks, because unless a writer knows how to structure English prose, there is no way of deciding where punctuation marks belong in order to indicate the stucture for the reader. Punctuation is not essential; when Greek was first written downn, it had not been invented. However, the result was very hard work, and although the legal draughtsmen are wary of anything except the full stop, the rest of us like reading a lot more if what we are reading is well punctuated, so that we know where each paragrap[h is going at first glance, and never have to go back and read a section a second time in order to work out how a sentence is structured.
Mastery of punctuation should therefore be fundamental; but there is a huge lack of it, and indeed some supposed experts violently disagree with others, but usually on certain points, one of which is the semicolon.
I heard a BBC Radio 4discussion in whcih two university teachers of English said that they didn't trust undergraduates to use a semicolon properly. This is a council of despair, and I consider those teachers incompetent at their jobs.
Once setence structure is understood and there is a decent grasp of punctuation, the organization of thought is the next requirement. That is way beyond the topic of writing bugbears, but I have come across supposedly learned books by supposedly very learned people, whose thought processes are either to be marvelled at or despaired of, for their writing can be almost inpenetrable.
One example of this is Kant and the Platypus by Umberto Eco, and although you might say "ah, well, you just aren't clever enough to understand what he's saying" I think I am, and I think his thought process is just a muddle, and is a long way from understanding what he is trying to write about. I cannot develop that argument here; it is not the right place and I don't have the tme this year (and it is only March). But I shall establish my case one of these days.