Panda Updates are rolled out time to time in order to prevent on page spam. Google does not like misuse of on page elements that are purely embedded for increasing the rank on SERPs. Hence the company introduced a filter that could discover spam elements on the page and deal accordingly.
Content farms where the most targeted using this filter. Blogs and websites using inadequate contents in order to rank higher were pulled down. I had come across many blogs acting as content farms which were of very low quality written by novice writers. The articles were used to publish on blogs, scraper sites and websites in order to rank high and eventually benefit from the published Ads.
Google also targets such sites by making changes to its algorithms. The search engine giant has been busy cleaning up the mess. Many sites that have escaped the filters would eventually come under the net in future roll outs.
Hence the moral of the story is that one should have unique informative contents if you wish your site to rank well and attain impressive traffic. Misuse of on page elements like H1, H2, H3 tags, repeated keywords and irrelevant and shallow literature on sites will result in rankings going down.
Though it is anybody's guess as to what content the search engine ascribes as unique. But it is simple approach that matters. No copied contents, no internal duplicate contents, no verbosity or garbage. Be sensible in order to avoid online penalties which the search engines may impose.