An excerpt from http://www.sitepoint.com/mobile-friendly-design-12-tips-follow/, by @richajn
Though smartphones and tablets have been around for less than 10 years, mobile Internet access has already caught up with desktop access. More Google searches take place on mobile devices than on computers in 10 countries including the US and Japan.
Google has been talking of this trend for a while now, and recently made changes to their search algorithm, to boosts the rank of mobile friendly websites in search results and penalize sites that are not mobile friendly. Commonly called “Mobilegaddon”, many sites saw their rankings drop significantly on mobile after these changes, and countless others are scrambling to make their sites mobile friendly. Yet, we continue to have websites that just don’t work well on Mobile. Even new websites.
Search engine optimization isn’t just the domain of Internet marketers. Website design and its implementation has an impact on search engine results.
As designers and developers, we need to take a step back and evaluate our work. The websites we create can’t just look pretty. They need to serve the needs of the user and generate revenues for our clients. Don’t leave the “mobile-friendly” test to the end of your design. Incorporate it into the design and development.
To help you out, here’s a shortlist of the top questions you should ask to better understand how mobile-friendly your site is.