The change won't affect responsive websites that automatically resize to fit your device (like Engadget), and desktop-only sites are fine. The only serious concern is for sites that have separate content for desktop and mobile. Creators will want to check that they aren't neglecting their mobile sites and make them as easy to search as their PC counterparts.

Google makes no bones about why it's looking at such a big shift in priorities. Most people search from mobile devices these days -- shouldn't the index reflect the pages you're more likely to see? While the company has made numerous efforts to improve mobile search in recent years, it's at the point where it has to make fundamental changes if it wants to stay in sync with reality.