The material should be accessible to all, without seeming to patronise the more knowledgeable. For wordprocessed text, a readability measure can help you see whether you are pitching correctly. Beware of long sentences. Anything over 40 words should survive only if it is truly excellent. Beware of constant sentence and paragraph length.
Variety is the spice of reading.
Avoid the passive voice. Beware of jargon - either specialised words or specialised uses of everyday words. Steer clear of sexist assumptions - doctors can be female, nurses can be male. It should be big enough to be comprehensive, eg Chambers English, Concise Oxford. Beware of wordprocessor spellcheckers.