The principles involved with designing mobile sites can be boiled down to one word: simple.
Mobile sites have to be simple in design. People are viewing and interacting with them on small devices. It is for this reason that columns, sidebars, and complex backgrounds that work well on a laptop or desktop are uninviting and inaccessible on mobile devices.
The second dimension of "simple" when it comes to design for mobile devices is that sites can't be loaded up with plugins (such as Flash), server-side scripting (such as PHP), or complicated navigation schemes. Some of these features are supported in some mobile devices (Flash is supported in some versions of the Android operating system), but most are not. Moreover, mobile devices have limited processing power, battery time, and other constraints that take us back to the watchword: simple.
Laptops and desktops on one hand and mobile devices on the other have quite different interface features. Obviously, in most cases...