Many of us who have been in the business for some time have watched, and worked, with interest as prominence has been added, and taken away from, key SEO factors.
First, it was the importance of on-page optimisation, which became subject to major abuse with issues such as spamdexing, keyword stuffing, doorway pages and so on. Google changed its tactics and we saw far more relevance given to links. That too has become subject of abuse and is now less relevant in some ways, although the importance of back links etc is never going to cease entirely to be a constituent of the algorithms.
Now, it seems that behavioural analysis may be the new black amongst the search engines. Great article here describing how Google collect personal data from users, which should encourage SEOs to think about how they can use such information. And perhaps endeavour to learn from the past and use, rather than abuse, the system.
Meanwhile, your average surfer should, maybe, contemplate the implications of Google holding so much personal data.