Update (17.11.2015): The new Searchmetrics mobile ranking factors study came out. Check out the results in our updated blog post.
Influencing Factors on Mobile Search Results
These are the most important parameters influencing the mobile search results:
Device: The device has an impact on the displayed search results. For instance, if you search for an app on the iPhone it is more likely that you will find an App Store result, whereas Play Store results are more prominent for searches on Android devices.
Location: The impact of local parameters is greater on mobile devices. If you are a local business you should have a look at the ranking factors for local search.
Mobile URLs: A lot of domains provide the mobile version of their website on a separate URL (e.g.: m.example.com). Accordingly, these mobile URLs appear in mobile search.
Ranking Factors for Mobile vs. Desktop
These are the most important differences in the ranking factors between mobile and desktop discovered by the study:
Content: The search results in the top positions in mobile search contain shorter texts and less flash elements. At the same time, the content is richer and features a higher number of semantically related terms.
Page Load Speed: Pages appearing in mobile search load a tenth of a second faster on average. Thus page speed is even more important for mobile search.
Backlinks: Although backlinks are relevant for mobile search results as well, the results on the top positions have less external links than desktop results. The reason for this is that mobile URLs get less links than desktop URLs.
Social Signals: The visibility on social media is equally important for mobile like for desktop. This is linked to the fact that people use social media as actively on mobile devices as on the desktop computer.
For detailed information you can download the Searchmetrics study for free (in German).
Update (2.3.2015): Google has officially announced that mobile-friendliness is going to be a ranking factor starting from April 21 2015.