The end result will look overexposed in the center and underexposed on the edges. Bigger is better. For increased legibility, increase your font size to a minimum of 18 points. Bolder is better. Whenever possible enhance the legibility of your type by making the typeface bold.
Contrast adds clarity.
Select colors which enhance contrast with the background. Upper case type is normally more difficult to read and may add confusion for your audience. Limit the number of typefaces. Try to limit the number of typefaces to two or three in your presentation. Serif versus Sans Serif.