For a business website to be truly effective, it must be mobile responsive. Your customers should never have to pinch and zoom their mobile or tablet screen in order to properly read or navigate your site.
Only a few years ago even good websites had a mobile version (the URL prefixed with m. ) and a desktop version. Now, with over half of all internet traffic accessed via mobiles/tablets, there are strong reasons to have a fully responsive site that works well on all devices, but is optimised for mobiles. (There are still instances where having a m. prefixed site is justified, for example YouTube).
You may be surprised that a site that was built recently and looks good on a desktop/laptop may actually not be fully responsive.
A simple test is to try out your site on a mobile. Navigate around, try all pages, functions and contact. View pictures, click menu options, click on links, use maps. If it doesn't work smoothly, get it resolved - fast!
One of the core elements of our rating service is to check a site thoroughly on a wide range of devices (inc different brands, sizes and browsers).
I'll have more blogs on mobile first websites, as there is so much to consider (for instance data consumption, location services, etc.).