Mobile sites and apps are sometimes the reason why I browse the desktop version, even on a mobile device. The mobile site usually presents a very shallow selection of the site's content. The same goes for mobile apps. While they are undoubtedly better than the site, they often don't allow you to browse or search very far back.

The best mobile sites are the ones that aren't mobile at all and just make proper use of CSS and whatnot to adapt the viewport.