A brief search engine algorithm history
In 2006, I wrote an article on which we've built the foundational principles at Anvil & Formic Media: "The 3 Cs of SEO." In the article, I outlined the three essential components of a sound SEO strategy: (quality) content, (clean) code, and (link) credibility. Although the Panda and Penguin updates are significant advancements, companies that adhere to the "3 Cs" have not been adversely affected by algorithm updates. In fact, sites with properly implemented SEO have experienced a recent increase in rankings.
Each year, SEOmoz "Ranking Factors" outline what SEO pundits believe to be the primary focus of search algorithms today and in the future. The past few years, those elements, in order of importance, include page and domain level link metrics, keyword usage, and social and brand metrics. Below is a chart from MarketingSherpa, which outlines various SEO strategies and tactics, based on degree of difficulty and effectiveness:
Pundits believe future rankings will evolve to focus more on the user experience: perceived user value, social signals at page and domain level, usage data (CTR, bounces), and content readability/usability. This is good news for marketers who've stayed true to the "3 Cs," as well as good design. In fact, I believe this is a significant step toward a convergence between optimization for search and user experience, which is good for everyone, especially the end user.