Google is not very explicit about this, but threatens to downgrade poorly optimized sites for mobile websites. With a few guidelines, it encourages them to take steps from their (webmasters) smartphones, available on the site here, to avoid mistakes.
The new guideline made by google is “to improve the search experience for users of smartphones” and meet some of their requirements. Google will indeed make changes to the listing to reflect sites that are “misconfigured” for smartphone users.
The threat of decommissioning is veiled and adds to the fact that in the coming months, the search engine will also take into account the speed of the loading of mobile sites for its ranking.
Among the mistakes to avoid, Google highlights the bad redirects, optimized for smartphones, and cites as an example for all pages on a site that redirects mobile users to the home page and not their real equivalent site.
A practice that irritates Google when mobile user is entitled to a 404, where there is no page optimized for mobile view. For more information, visit this page, dedicated to common mistakes with sites for smartphones.