Alignment. Nothing should be placed on the page arbitrarily. Every element should have some visual connection with another element on the page.
So very true, this quote from Robin William's book The Non-Designer's Design Book. Architects such as Le Corbusier are the masters in planning the proportion and alignment of objects on a facade.
The above image and quotes were taken from page 26 of Le Corbusiers's The Modulor 1&2. (Click on the page 26 Google books link or the image to read the text)
Not that you have to be like them. But still remember from now, whenever you have the option to position a shape or a text box on a slide, why not think for a second where to put it, align it with something?