A.K.A. Farmer's update, as referred to by SEOs who believe this change was made to de-emphasize websites with more ads than content. There are many unhappy webmasters after this update, who have seen their traffic plummet. In fact, 12% of Google searches have been affected by this change, but there are steps you can take to minimize the impact on your website.
1. Make sure each page has quality content, no cutting and pasting unedited from other sites. Rewrite the content so that it is seen as unique. Try to go deeper into your subject and provide useful links.
2. Remove any pages that can't be rewritten to provide quality. A few bad pages can bring down your ratings.
3. Remove duplicate content. Make every page count.
4. Don't make visitors register to see content; this will bring down your ratings.
5. Well designed (pretty) sites hold your visitors’ attention longer, they will stick around to read more and this improves your ratings.
6. Links still matter. Make your site easy to link to and request that other quality sites link to you.
7. Use Twitter. Links in Twitter improve your ranking. Also make sure to respond to other people, as you need to participate on Twitter to improve your traffic. Get others to spread your links. Again, this will only happen if you have unique, quality content. Put up an avatar or a portrait and use a Twitter name that doesn't include numbers. Also, complete your bio information and use a URL that differs from the one you are promoting. Don't forget that tweets go 'stale' so add some new content every few weeks (or better yet, blog and encourage comments) and retweet those links.
8. Put content before the ads, as in higher on the page. The Google Page layout algo will penalize you if the ads come first.