So says Idan Gazit, about 3/4 of the way down the page in his post on Designing Presentations.
Forcing yourself to ruthlessly trim your slide content is the best way to evaluate what really matters to your message. Can you cut the slide or merge it with another one? Is there a better combination of words to illustrate the point you’re making?
Less content also has another nice side effect: you can make your text large. Enormous is good—people in the back are often quite far from the projector screen. Make their lives easier and they’re more apt to give you their attention.
Don’t bother with clipart, animation, and other little visual embellishments. Better to have something clean, typographical, and timeless than a powerpoint cliché.