For some time, I have been wrestling with why I was unable to communicate as effectively as I wanted to using PowerPoint, which, for better or worse, has become the de facto standard for visual slide-based presentations in modern corporations (and it even gets used in high schools - both of my children have already been exposed to PowerPoint in their curricula).
It's easy to see folks presenting using PowerPoint in a way that reduces their credibility (the most obvious sin being to simply read text off a foil, implying that the audience is too dumb and/or too lazy to read it). I have seen a number of other egregious abuses over the years.
However, last year I spent a modest sum of money on a document from Edward Tufte, one of the thought leaders in the art of visual presentation of information. He wrote "The Cognitive Style of Powerpoint" late in 2003, as a way of alerting us to the many dangers lurking beneath the surface of that software tool. The paper does an excellent job of explaining the issues with the use of PowerPoint (far better than I could), but here are some of the main points that Tufte makes:

- The text-based approach to creating foils leads to appallingly low information density - far worse even than the spoken word
- The hierarchical style of bulleted indented lists leads to the trivialization, artificial simplification and unselective editting of information
- Normal use of large-resolution tables elminates any serious quantitative analysis of data, replacing it with out-of-context editted summaries that may seriously distort or falsify messages
- Use of clip art ("Phluff" is how Tufte refers to it) further reduces information density, patronizes audiences and bores serious listeners to tears

Tufte illustrates many of the issues with examples from real life, most notably from the report on the demise of the space shuttle Columbia in 2003. The centrepiece of his document is an absolutely appalling PowerPoint slide submitted to the Accident Investigation team that completely distorts analysis results, as the foil moves from a bland and corporate double-speak high-level summary that appears to suggest no significant risk, to poorly-presented lower level information showing that there is a very significant risk to the shuttle on re-entry. He also includes an amusingly-mangled version of the Gettysburg address converted to a classically awful corporate-style PowerPoint presentation, all created using PowerPoint's Auto-Content Wizard. I have to say that I never used this feature, and now I see what it could do to a message, I'm glad I never tried it. I shudder to think what you could do with the Declaration of Independence or the Preamble to the Constitution...
"The Cognitive Style of PowerPoint" may be the best $8 I ever spent on a document...