If what happened to about 50 of my pages in different niches yesterday is typical, the search result quality of Google has overnight worsened by at least 80%.
For one of my insurance sites, all pages with quality content are gone, but my disclaimer page now appears at the 4th page for a pretty competitive keyword.
For another site where I review learn guitar programs, my 'guitar program X review' page now ranks for 'guitar program Y review'.
For many Wordpress websites I have, only 'search' and 'tag' pages are left. They just came up out of the blue and rank well since yesterday - cheesy 'search' pages Wordpress seems to autogenerate and Google picks them up - I didn't even know these pages existed, and they have due to their nature of course crappy content compared to the 'real' pages.
Generally, in my case it seems to be an 'anchortext' penalty.
Basically, all my home pages are gone, but inner pages with little to no backlinks rank for keywords I had never targeted, sometimes for completely irrelevant keywords.
The quality of content and whether it's duplicate content or not does not seem to make a difference in my case. The opposite, my worst sites went a few positions up, and my high quality unique sites are wiped out. Sites with just one or two pages, like those I had just set up with some duplicate content thrown on there for now, went up. It's ridiculous.
Way to go, Google - this change is going to make a real improvement in your users search experience.