May 22, 2013,in US,the next generation of the Penguin webspam algorithm was rolled up,which have caused a stir in SEO field.I think ,the following kind of webstes are most likely to be affected:
1 . Excessive optimize business keywords
2 . The proportion of nofollow link is too small
3 . The proportion of text links and blog link is too high
4 . The proportion of inside pages link is too high
5 . A lot of links from the black site or sites containing malicious software

This is my suggestion.Do you have other better analysis to share?