Some presentation slides do convey so much more than others. More often than not, these are same slides that are simple and visual at the same time. By simple and visual, I really don't mean that you put in a box or a circle and add some captions -- any shape including boxes, circles, and other shapes have to form a relationship with each other so that difficult concepts transform to easier ones.
And such slide designs are not something that everyone can create -- these do require an understanding of visual balance and layout. Fortunately, vendors like PowerFrameworks makes it easier for you to access such visuals.