A few notes on written communication

Do Not Hand Wave over Important Topics

The gift of written communication outside of Twitter and Nicheless is that the word count is generally boundless. We can dive into any point and pull our selves back out to any level of abstraction that we want to. When and where to dive into writing is art, not science. But such as it is with any communication, my suggestion is to, first, overcommunicate, and, then, remove, if necessary. I cannot tell you how many times I have seen a project proposal written with "[this feature] will lead to increased revenue" or an engineering document that says "we will make calls to [some service]." In some cases this might be ideal, but out of the gate, I prefer to see the details that support these broad statements.
Beware the Curse of Knowledge

Engineers are very technical and a lot of time we have blinders on to who our audience is. We use jargon, abbreviations, and memes. All of this is insular, exclusive, and has an expiration date. The worst is when a document is full of domain language that isn't referenced anywhere. This is the curse of knowledge. I assume the author has the same information that I do. CoK is the kissing cousin of Hand-Waving over topics. The suggestion, here, is to take time to either provide supporting references to the information, or to deconstruct the big ideas into small palatable ideas.
It doesn't mean we must build everything from first principles, but if we can get close, so much the better. For specific audiences, we can certainly assume a certain level of knowledge, experience, and subject matter expertise. Even then, it might be considerate to include an "Expected Audience Experience" or something to that affect.