In 2009, Google launched a feature that let you filter your search results by web pages you have not yet visited. That feature went offline temporarily two years later. Google is now calling out the feature on Google+ saying this filter will help you "search more efficiently."
Here is a picture on how to use it:
Google says that you must be logged in for this search to work. Clearly, Google is using your Google Web History to power this feature.
I think Google is calling this feature out to see if more people will use it.
The main issue, people aren't clicking deeper into the search tools. The old user interface showed the search tools and filters on the left side. Now they are buried beneath the "search options" button and then these drop downs. Yes, the results are cleaner and most searches won't bother filtering or sorting search results, but I bet usage dropped.
As an SEO and marketer, I bet you can figure ways to use this filter to benefit you.
Forum discussion at Google+.