See Principles of Visual Design from the Nielsen Group for more information.
The difference between two sections. For example a light background with dark text.
Reusing the same fonts, colours or shapes.
How content is positioned on the page. Is it all balanced on the page? Is the text left aligned, centered or right aligned.
How close are object to each other.
When you have two pieces of text you will want a gap so that the text is easier to read.
Small changes can have a big impact.