My list of five guidelines that are important:
1. It's imperative never to read off the slide presentation. This doesn't substitute your presentation.
2. Simplicity and briefness on your slide makes it easier to understand for everyone. Too much information will overload some.
3. The slideshow should have no significant value without your narration behind it.
4. Good visuals that connect to your presentation will connect to your audience even better.
5. A lot of color can be a good and bad thing. Try to color code, it's easier on the eyes, makes for better organization, and adds clarity.
A lot of writing on slides is really frustrating and the first thing I think of personally, is sleeping.
Statistics are another thing that bores me to death. Give me one and move on. Nothing worse than years of numbers a mile long.