Someone at Google leaked the document titled Quality Rater Guidelines (go read it). The document details how quality raters review search results and also details what Google considers spam. We will not go into detail about the document in this post, but it shows that raters are very real.
Unlike algorithms, you cannot trick human review, so make sure that your content reads as naturally as it possibly can, because once you're flagged by a human, it's very hard to get your position back.
Behavioral Data and Personalized Search
Behavioral data analysis includes click-through rate, time spent on each site, pages visited and more. Google and other search engines monitor and analyze this data. On top of the tracking listed above, Google is likely to be monitoring users with a long history of specific searches.
Personalized Search
If you have a Google account and use Google search on a regular basis, it adjusts search results based on the sites you visit the most, search queries you frequently use, blogs you track with Google Reader and other content you find/read using Google services. It uses this data to bring you partially customized results that are different from its regular results.
With this in mind, we can hypothesize that once Google has enough data from a pool of users, it can cross-compare and determine if a number of users with specific topics of interest share the same site-visiting patterns. If 1000 users interested in SEO read "SEO Chat" via Google Reader, then it's a good indicator that SEO Chat is an important website.
Based on collected behavior information, Google can adjust its search results. I would say that they would not go so far as to determine search results based on behavior, but they definitely use it to adjust and improve SERPs.
Microsoft Browse Rank is another example of how search engines use behavioral data. With the focus on behavioral data, now more than ever you need to build your site with your human visitors in mind. After all, that's truly the point of Google's algorithms: to return sites that human searchers will find relevant.
Google Optimization 
