Here's the Gettysburg Address presented as a series of PowerPoint slides: a cool example of why you shouldn't use PowerPoint to communicate everything. The related essay is interesting.
One thing that really annoys me in my work life is the way that people sometimes think that every meeting needs to be scripted with meticulously crafted PowerPoint slides. People hide behind the messages on the projector screen, rather than cutting to the chase and having a really productive, dynamic conversation. People copy slides from the corporate deck and recite them slavishly, without thinking about exactly what is required for the meeting at hand. The worst meetings are often ones where people actually read word for word from PowerPoint.
This is not to say that PowerPoint is a useless tool -- I actually find it very useful. But it's good for particular things: punchy summaries, providing structure to complex discussions. It shouldn't be the detailed script for a whole meeting.
(via Matt Cutts, where the Gettysburg link was mentioned as a tangent)