1. Violation of Google's webmaster guidelines : Overall theme of Google's webmaster guidelines is to not attempt to trick the search engines. For enhancing SERP results and PageRank, the avoidance of what Google calls "link spammers" and "bad neighborhoods".
2. Link farms : A link farm exists solely to increase PageRank by requiring the exchange of links between themselves and otherwise entirely unrelated websites.
3. Hidden text or Hidden links : It also disapproves of "cloaking" and "sneaky redirects". Hidden text is usually in the form of keywords, written very small, usually in the same color as the web page, or both. The idea is to have the text read by the search engine spider, but not by the site visitor.
4. Heavy use of keywords, that clearly do not belong in the context of the web page.
5. Duplicate content, whether pages or entire websites is specifically against the prescribed guidelines. Identical pages are used to add more pages of content.
6. Doorway pages, that are packed with every imaginable keyword to attract the search query, and then steer that traffic via a "sneaky redirect" to another site are expressly forbidden.
Source : http://research4seo.blog.com/