Be authoritative without being pedantic. At the same time, I've tried to avoid burdening the reader with unnecessary detail. Be organized for easy learning. My experience in teaching C underscores the importance of presenting the features of C gradually. I use a spiral approach, in which difficult topics are introduced briefly, then revisited one or more times later in the book with details added each time.
Pacing is deliberate, with each chapter building gradually on what has come before.
Motivate language features. Instead of just describing each feature of the language and giving a few simple examples of how the feature is used, I've tried to motivate each feature and discuss how it's used in practical situations. It's important for every C programmer to develop a consistent style. Rather than dictating what this style should be, though, I usually describe a few possibilities and let the reader chose the one that's most appealing. Avoid dependence on a particular machine, compiler, or operating system.