Here are some of the aspects that Google will be assessing in deciding how mobile-friendly your site is:
- Text must be easily readable on mobile devices. Fonts should automatically resize for the smaller screen. Non mobile-friendly sites simply squash the standard desktop site into the smaller screen space of the mobile browser so everything appears quite small and pretty much requires a lot of pinching to zoom in order to read text or interact with page elements.
- Any clickable elements like hyperlinks or buttons must be properly spaced so that they are don't throw the visitor into a hellish fat finger pit.
- Dynamic content must continue to work. You've seen those site that won't play video or a slideshow on your tablet or smartphone (most often because the content is Adobe Flash based). That's a no no. Such content must be converted to work on mobile (e.g., HTML5 video).
- Vertical scrolling is okay, horizontal is not. Page content needs to fit into the viewing screen so that horizontal scrolling is never required.
- Each page on your site will be assessed for mobile-friendliness so, should you have any non mobile-friendly pages, they'll be flagged.