Every day, I run into people affected by the Panda update. "Affected" always means negatively affected, because when your rankings go up, it means that Google is finally recognizing the quality of your site. When your rankings go down, blame it on Panda. Now, none of this is to say that Google's Panda algorithm update, and its ongoing tweaking that has followed, is not a big deal. In fact, this might be the biggest search ranking algorithm change since Google inaugurated link analysis in the late '90s. Because the search ranking algorithm isn't really analgorithm any more, because people don't set the rules. Google has embraced machine learning, and SEO will never be the same.
Machine learning is a computer technique that uses pattern recognition to cluster together similar things. What Google is doing with machine learning is clustering good search results and separating them from bad search results. How do they do that? They have people working as search quality raters to decide which search results contain quality information and which doesn't.
Those ratings become the training data for machine learning. Then the machine learning techniques go to work, identifying which Web pages and which entire Web sites have qualities similar to the ones that the quality raters have judged the best--and also noting which sites seem to match low-quality sites. Then, in addition to all of the factors that Google has always looked at in its ranking algorithm, there's a new factor--this machine learning factor based on the search quality raters.
So what does this mean for SEO? Plenty. If you've been trying to reverse engineer the algorithm, you can stop now. If you've been playing tricks with content farms or paid links or any other spammy technique. stand down. Those techniques are probably not going to work very well for much longer. I say that not because the new algorithm is perfect. It's not. I've seen plenty of quality sites that have been unfairly downgraded. I am saying it because Google is committed to this approach and will keep at it until they get what they want. They've declared war on spam and finally have a computer technique that can add in a human judgment that is very hard to spam.
So, if your idea of SEO is gaming the ranking algorithm, then SEO is over. If your idea of SEO is providing the best content for searchers to find, then your time has come. I've always counseled companies to optimize for people more than for search engines. Never has that advice been more important than now