This is a very important aspect, if not the most important one. The visual design sorts the wheat from the chaff and distinguishes a good presentation from an average one.
The worst presentations are the ones which are extremely text-heavy. Why don’t you use Word if you want to write a novel?
Average presentations contain some visual elements but don’t excel in creativity and design.
Great presentations, however, have sophisticated visual elements on every slide. Not for their own sake, but in order to effectively underline and transport the content and the thoughts of the creator to the audience. It is scientifically proven that people remember much more if they not only hear information but hear and see it. So use this effect and visualize as much as you can. I could go on for hours writing about the visualization of information but I think it is more effective to show you some striking examples. So just take a look at the example slides and you will immediately see the benefit of visualized information.
Check the following examples: