I believe writing today for a popular audience means, among other things, not writing as if every reader has the same learning style. Whether due to a lack of aptitude, the press of circumstances, form of education, or some other reason, many people are not particularly adept at tracking closely with an extended argument presented in big blocks of prose. (Most authors and editors do this for a living, so we don't notice.) We must not write as though the reader is unintelligent or childlike, but if there is one kind of person I generally “edit to,” it is the busy parent who only has (or only makes) 20 minutes on a Thursday night to read this kind of book on the couch just before bed. He is tired. He is likely reading this book in part out of a sense of obligation, and he only vaguely remembers the six pages he read three days earlier at lunchtime. How do you serve him? You don't need to dumb down the content. In addition to good, clear, direct writing, you need to use structural elements like heads and subheads to highlight the sub-points of the teaching.
Secondary and tertiary subheads, with the occasional list of smaller one-off points, help the reader in two ways. They allow him to see the logic of the argument unfold forward, not just in blocks of prose but with visual reinforcement (for visual learners). They also allow him to review the recent content backward with the same ease. He can flip back a couple of pages and—without having had to mark up the text as he goes—be able to recall quickly the gist of the author’s argument.
Two common techniques that rarely serve readers are 1) paragraphs that begin with “First,” “Second,” etc., and/or 2) a single italicized word buried in the middle of a block of text and intended to serve as a label for the central concept of that section. To me, a reader of a popular book should never have to mark up a chapter to be able to scan for the basic elements of the argument. We should not ask readers to do more than purchase a book in order to own something that rises to a baseline level of ready intelligibility—and (this may be flipping the metaphor) I believe that bar has been raised several notches in just the past several years.
People expect high-quality content. Given the influence of the web and the pace of life, they increasingly expect that content to be as easily digestible as the concepts will allow. Editors and publishers owe readers: quality content + ready intelligibility, which = surprise and delight. In exchange for their cash and attention we owe them excellent material, as accessible as possible, without dumbing down or compromising.
Quality, plus ready intelligibility through a structure that serves more than one kind of learning style: I believe this is what popular communication means today, and this is what people expect their money to buy them.