Creating icons are hard; interpreting icons can be just as hard. The guide should have an elaborate section that can satisfy both parties so that it's easy to design visual metaphors that can be quickly understand them in their user interface context. There are some basic guides that should be followed, such as:

1. avoid culture-specific symbols. For example, a four leaf clover might resemble luck in some cultures, but outside that might not be understood.
2. avoid abstract concepts, try to create concrete objects as icons instead.
3. Sometimes icons will need labels with them to show the meaning. There should be some discussion on deciding when an icon will need a label :)
4. There's also a section in the old WikiFont archive page that we can still apply, which is "Avoid concepts such as gestures, animals, religion, humor, ethnicity, and gender." The direction section can apply too.