I've learned so many by being in a critique group. Being privy to the writing efforts of a couple dozen people on a regular basis is a treasure trove of little bits and pieces about the way others write.
One of the greatest problems I've observed in some writers is that they choose a subject and then they ramble about it and six other things, as well. I think it comes from the way our minds take us. One thought leads to another, but readers aren't going to have the patience to wade through your ramblings.
Once I read what I thought was a promising piece of nonfiction, at least at the beginning. The writer let me know in the opening paragraph what the subject was, but by the time I'd gotten to the end (which took an amazing amount of words and my time), I had no idea what her theme and/or thesis actually was. She'd rambled on and on at the beginning, then switched to an entirely different subject and ramble on and on about it, and then on to yet another. By the time I'd finished reading, my head was whirling with wonder. Wondering what the whole piece was about!
When you proofread your work, an important part of your checklist should be to ask yourself if you have stayed true to the subject matter. Did you stay on track, or did you take an entirely different set of paths through the whole piece? It may be OK to ramble on in your own thought process, but stick to the subject in your writing.
Fiction writers need to be wary of this, too. Let your characters do only what is improtant to the story line. I've read novels where I've thought to myself, Get on with the story! Enough of all this extrra drivel.
How many times have you read in a writer's reference book that you should "Write tight?" Maybe they should add "Don't ramble." Might be a bit redundant for they both come down to the same thing, but it could make a few writers open their eyes to what they need to do to writer marketable nonfiction and fiction.