Unclear Fonts & Colours
Some colour combinations simply do not work well in print. Saturated colours in particular do not work against most backgrounds. Script-type fonts should be used with care as often the strokes are too thin to render well when printing.
Type should never be less than 8pt unless there is a clear reason (such as space considerations) and then fonts created for the purpose of small type should be chosen.
Font colour should always be solid and never a shade. For example, if text is meant to be gray, do not make it black at 75%. The printer will render it by printing black but skipping every forth dot which can leave the edges of the type looking ragged. Choose instead an actual solid grey colour to obtain smooth printing.
Contrasting colours fight with each other for dominance, causing the text to appear to "float" off the page. Thin fonts can be difficult to read. A font colour that is too close in value to the background colour starts to disappear.
Dark fonts against a light background and the reverse, and avoidance of thin scripts make documents easier to read.