In a recently posted article on the official Google blog called “Another step to reward high-quality sites” explains and gives and example how anchor text is one of the more important factors for the Google Penguin update. That is later confirmed by Matt Cutts and the industry itself. But, the way things are the importance of that anchor text may be significantly higher than what was thought at the beginning. Namely, in two examples where they mention domains that have been caught in the Google Penguin update they mention anchor text as one of the most important factors for the stated topic.
In the post “How WPMU.org recovered from the penguin update” anchor text is mention as the main perpetrator. Same, in the post “Recovering from an Over Optimization Penalty – A true story” they also mention anchor text as one of the main perpetrators. From the stated stories it can be easily concluded that the first thing that should be checked are anchor texts that have been used during link building. If they fit too much and too often to the words you want to rank for you will probably have problems with the Penguin update. Check your anchor texts and make them more natural using the tips from the aforementioned articles.