Below are a set of guidelines (see discussion) for creating new content and revising existing content.
1. Specific before general, concrete before abstract Students don't learn by being given the most general (and most powerful) equations and procedures, they learn by generalizing specific (and less powerful, and less general) procedures themselves. Pattern recognition is one of the greatest human strengths, memorization of general formulae is not. Specific examples should be used to illustrate general principles, and then those general principles can be outlined.
2. Simple before complex When teaching a new concept, the simplest possible example with the fewest extraneous details should be presented. When introducing resistors, we should talk about a single resistor instead of 2 or 3 (or, god forbid, an arbitrary number) of resistors. No unnecessary detail should appear before it is absolutely necessary. "If in the first act you have hung a pistol on the wall, then in the following one it should be fired. Otherwise don't put it there." - From Gurlyand's Reminiscences of A. P. Chekhov
3. Concise over verbose "Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all his sentences short, or that he avoid all detail and treat his subjects only in outline, but that he make every word tell." — "Elementary Principles of Composition", The Elements of Style
4. Motivate and tell a story, don't present facts Why should we care, anyway? Facts aren't interesting, no one cares about your facts or what you have to say. Facts must be motivated with stories. Humans remember, and learn from stories. Content should be a narrative centering around a few core ideas which you should clearly motivate and articulate. Content should use personal/historical stories whenever possible, should make use of real-world examples, and should always build on itself. “Storytelling is the most powerful way to put ideas into the world.” – Robert McKee
5. Make stuff easy to find. Put the core ideas at the top and the bottom of the page ("Top line" and "Bottom line"). Add references at the end of each big section (chapter?) for anybody who wants to go deeper.
6. Always go through the derivation. If you don't know how it's derived, you don't know how it's defined. If you don't know how it's defined, you can't do jack squat. This should be consistent with the above principles (in particular Simple before Complex and Concise over Verbose).