Incoming Links

Incoming links should be:

• Related to your site
• Not link to too many other pages (it dilutes the flow-on of their ranking to your site)
• Have a reasonable Page Rank

http://www.ssw.com.au/SSW/Standards/Rules/RulesToBetterGoogleRankings.aspx#SimilarSites

Search engines may penalise your site if it is part of a "link farm", because they just add to the mess of websites and reduce the overall quality of the Internet

http://www.ssw.com.au/SSW/Standards/Rules/RulesToBetterGoogleRankings.aspx#LinkFarms