The dilema of providing an improved user experience by presenting the most authoritive articles first; and the difficulty of establishing which articles are the most authoritative is something that Google does very well. In the past the number of links going to an article provided a measure of how authorative an article was, but this was open to abuse and some web owners purchased low quality links and directed these to their page.

This practice of using low quality links was detrimental to the authority of the article and Google then started to penalise articles for poor quality links or unnatural links. However, this too became and issue when black hat seo practitioners used the low quality links as a means of dicrediting good articles in order to promote their own clients.

Google consequently implemented systems to detect patterns of unnatural, artificial, deceptive, or manipulative links pointing to the site. When such lnks are identified, Google advises the webmaster through a 'Unnatural links to your site - impacts links' message on the Manual Actions page on Google Webmaster tools.

The site owner then can use Google's Disavow tool to remove any of these detrimental links.

 

Here is what Matt Cutts has to say about unnatural links - http://www.youtube.com/watch?v=7Y74Op_k6UY#t=95