There has been a lot of discussions abut penguin update (just like any google update) personally I don't think there is anything specially about it (unless you have not been using strictly white hat technique to promote your site. A list of some of the stuff Penguin frown upon are:
There are 8 “specific guidelines”. They are (verbatim):
1. Avoid hidden text or hidden links.
2. Don’t use cloaking or sneaky redirects.
3. Don’t send automated queries to Google.
4. Don’t load pages with irrelevant keywords.
5. Don’t create multiple pages, subdomains, or domains with substantially duplicate content.
6. Don’t create pages with malicious behavior, such as phishing or installing viruses, trojans, or other badware.
7. Avoid “doorway” pages created just for search engines, or other “cookie cutter” approaches such as affiliate programs with little or no original content.
8. If your site participates in an affiliate program, make sure that your site adds value. Provide unique and relevant content that gives users a reason to visit your site first.