Why Consistency is Critical [Usability and Information Architecture]

There is little doubt that consistency is important for users. Consistency makes sites easier to use, because visitors don't have to learn new tricks as they move around.

Sites should be internally consistent: standards and conventions should be established and applied throughout all the content. For example, a user who encounters the "Search" at the top right on one page will have problems if it's arbitrarily moved to different locations on other pages of the site.

Sites also need to be externally consistent, that is, consistent with general practice. Users will tend to apply rules they've learned elsewhere, even if those rules don't actually apply to the current site. They bring to your site their own experience and expectations. If you ignore that, you risk causing confusion and alienation. (Via Sitepoint)