Alan Jacobs, providing some of the best writing advice you’ll find:

Writing that matters will therefore be in service to something or someone, and in order to serve well, you must undergo training and discipline. You have to learn things. You have to have a full mind as well as a lively heart. First you must develop the expertise of crafting sentences, and this can only be done by, first, reading and reading and reading. There is no other path than hard-earned expertise to producing anything that’s worth the time and attention of readers. Why should anybody care what you think? You must earn their care through especially vivid writing, or especially clear thinking, or especially detailed knowledge — or (ideally) some combination of the above.

I put the last line in bold because it’s such good summary of what makes for great writing. I’ve started to subconsciously assess what I’m reading with these categories in mind. Excellent writing weaves together: 1) great prose; 2) clear thinking; and 3) detailed knowledge of the subject. When I’m reading something and discover that it fails to possess even one of these components, I try to put it down as quickly as possible. Incidentally, this helps to explain why so much theology that I’ve read, though factually correct, receives my readerly judgment in the form of setting the book aside. Would that more theologians married these three things together. There are, to be fair, some (happy) exceptions.