According to Ben Gomes in a post at the Official Google Blog, they are conducting “little experiments” and testing of everything from the barely visible to the glaringly obvious.” One of these experiments involves manipulating the search results not by Google’s automated machines but by Google users. In the screenshots, the green up and down arrows were being displayed at the end of every search results links. Clicking on those arrows influence future placing of the search results in Google SERPs the next time they are displayed.
Simply put the new feature is pointing towards a more sociallly human-powered search engine. This feature is of course still in experimental stage. But if indeed, Google plans on implementing this in the future, Google should put up something that would prevent the system from getting abused.