As of April 21, 2015 Google is using the degree to which a web site changes form based on the device being used to display it. “Mobile search” (from a phone or similar device) will get lower ratings if it is not mobile friendly; responsive. Google has a web page that will tell you whether a site meets their new criteria or not. Of the WordPress sites I’ve worked on recently, only two passed this test on first try – one done using wordpress.com and another done for the NEK Collaborative for which I used the Suffusion theme for the first time. Evidently it has responsiveness baked in well enough to pass Google’s tests.
To get this site (solvt.com) to pass Google’s test, I added the WPtouch Mobile Plugin. It takes a site not ready for mobile and uses a separate theme when it is displayed on a small screen. I used the default “Bauhaus” theme. Other themes are available with paid versions of WPtouch.