I think the logo should be simple, unique and easy to recognize. It should not contain too many colors and it should look good on both black and white background. Preferable some text together with a logo/illustration. Amazon has a simple logo with text and an arrow indicating that they sell anything from a to z. Other examples includes: http://www.canaldigital.com/,http://www.energinord.dk/,http://www.dsb.dk/