Hi,

Google is looking at penalizing ad heavy sites that make it difficult for people to find good content on web pages, If you have ads obscuring your content, you might want to think about it," inferring that a if a user is having a hard time viewing content that the site may be flagged as spam.

This is also a prime aspect of Google Panda Update along with the content related problem.

Quote from: jeffshawn on November 15, 2011, 09:22:07 AM
Hi,

Google is looking at penalizing ad heavy sites that make it difficult for people to find good content on web pages, If you have ads obscuring your content, you might want to think about it," inferring that a if a user is having a hard time viewing content that the site may be flagged as spam.

Ya that's 100% true.!
Also, its recent change impact on the sites which gained backlinks from paid link..

That for sure, one need to have good navigation as well as good content on their site as well as the ads should follow some rules suggested in SEO rules and regulations