What's all the fuss?
Some of it is just plain scare for scare sake, but the fact of the matter remains that if you have a unfriendly website when viewed in a mobile or tablet device, your rankings for MOBILE SEARCH RESULTS on Google will be affected by these changes.
Starting April 21, we will be expanding our use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results. Consequently, users will find it easier to get relevant, high quality search results that are optimized for their devices.
What about my Google ranking?
So your ranking in general wont be affected, it's not like you are going to drop off the Google index, remember your website is Googles bread and butter. But the update will affect ranking for non-compliant mobile websites, when searching on a mobile device. The exponential growth in use of mobile / tablet devices for browsing the Internet, V's the old PC or Laptop, means the percentage of media now consumed by mobile phones / tablet devices, makes up 60% of total traffic on the web.
As you can see by the graph below, we are now past the tipping point.
"...and so there comes a time for change in the realm of cyberspace and its masters."
Why does this makes sense to Google?
excerpt from google webmasters blog
When it comes to search on mobile devices, users should get the most relevant and timely results, no matter if the information lives on mobile-friendly web pages or apps. As more people use mobile devices to access the internet, so Google algorithms have to adapt to these usage patterns.
In the past, they have made updates to ensure a site is configured properly and view-able on modern devices. So now with this update, they have made it easier for users to find mobile-friendly web pages. An important change to help users discover more mobile-friendly content: