For years, the search engines have continued to introduce new factors into their algorithms to make their search results more relevant and to keep savvy search engine marketers from "cracking the system."
We've seen many ranking factors come and go in importance. For example, years ago, META tags were the key to success, or so we thought. Stick in META tags that were loaded with your keyword phrases, and you were sure to achieve top rankings.
Then, we had keyword weight as a ranking factor. We struggled to determine the keyword weight of our competitors' pages, then duplicate that weight in all of the various areas of our pages.