Google changes its algorithm on a regular basis, but most changes are so subtle that few notice. This is different. Google says the change impacts 12% (11.8% is the unrounded figure) of its search results in the US , a far higher impact on results than most of its algorithm changes. The change only impacts results in the US. It may be rolled out worldwide in the future.
While Google has come under intense pressure in the past month to act against content farms, the company told me that this change has been in the works since last January.