The above image is an example of a poorly designed slide with respect to readability.
A number of factors, such as room lighting and room size, can affect the readability of your slides during a presentation. Therefore, when creating your slides, choose font colors, styles and a font size that will make it easy for your audience to read what is on the screen, no matter where they are seated.
When changing font colors, choose ones that contrast strongly with your background. When choosing a font / background color combination, you might also want to consider the room you will be presenting in. Light color fonts on a dark background are often easier to read in a very dark room. Dark color fonts on light backgrounds, on the other hand, work better in rooms with some light.
In the case of font styles, avoid fancy fonts such as script styles. Difficult to read at the best of times on a computer screen, these fonts are almost impossible to decipher when projected on to a screen. Stick to standard fonts such as Arial, Times New Roman or Verdana.
The default sizes of fonts used in a PowerPoint presentation - 44 point text for titles and 32 point text for subtitles and bullets - should be the minimum sizes you use. If the room you are presenting in is very large you might need to increase the font size.