Here are a few things I believe in:
Function over Form
Ulitmately, a designer's job isn't to make things look good. It's to present information or create an experience that is as easy and seamless as possible for the user to understand. Design is always, always about the user.
Substance over Style
Content is king. With very few exceptions, people visit websites, or look at sales collateral, or read an ad, for the content, not the design. Design should enhance the content, but never so much that it overpowers the content.
Details, Details, Details
All this being said, form and function, style and substance - they're not mutually exlusive. A site or a book or an ad can (and should) still look great - paying special attention to the tiny details helps make that happen.