From a user experience (UX) perspective, consistency is a good thing (and not as the Emerson said “the hobgoblin of little minds”) (1). There are very few totally “intuitive” interfaces a so consistent look and feel allows the user to learn as they go, making it easier to navigate through the site and, as the user gains more and more experience on the site, predict where information can be found and how widgets, etc. will work.
That said, within a generally consistent visual and structural framework it is fine to allow the content to (a certain extent) dictate the style of the page. One example that comes to mind is varying whether a sidebar is available on a page. While a sidebar may be useful for listing archives and categories on a blog page on a site, it may take up valuable page real estate when displaying data or other information where there are not archives or categories available.
(1) Yeah, I know what he really said was “A foolish consistency is the hobgoblin of little minds” and when it comes to UX consistency is certainly not foolish.
