You do want one site to look like another site. I know if is fun for designers to make up their own UI's, but there are well established patterns that work well, and that's what everyone should use. This is how everything was when people built desktop applications because you used the OS supplied widgets. These widgets were built be experts in HCI. Everything worked the same in every app, and nobody thought about it or wished they were all unique. CSS and the web are the worst things that ever happened to usability.