I know someone who teaches speech who told me the two most important things about visual aids:
1. You should be able to see them.
2. They should help.
I attended a meeting recently where some of the presenters did not know this. There were slides which looked like a typical 8.5 by 11 page filled with text in 14 or 16 point font. I could not read them and they were often left up for too short a time for anyone to read them completely.
Sometimes legible slides had so much material on them, that if I read them, I would not be paying attention to the speaker.
Power Point can be used effectively, and some of the presentations were good. But one would think that these two basic facts about visual aids would be obvious and taken into consideration by people who plan to present to hundreds of people.