As previously reported, Google has updated its Webmaster Guidelines. Part of the update is some new stuff about rich snippets.
Google is highlighting these changes specifically in a post on the Webmaster Central blog today.
"Once you've correctly added structured data markup to you site, rich snippets are generated algorithmically based on that markup," Google says in the post. "If the markup on a page offers an accurate description of the page's content, is up-to-date, and is visible and easily discoverable on your page and by users, our algorithms are more likely to decide to show a rich snippet in Google’s search results."
"Alternatively, if the rich snippets markup on a page is spammy, misleading, or otherwise abusive, our algorithms are much more likely to ignore the markup and render a text-only snippet," Google adds. "Keep in mind that, while rich snippets are generated algorithmically, we do reserve the right to take manual action (e.g., disable rich snippets for a specific site) in cases where we see actions that hurt the experience for our users."
As far as the actual quality guidelines for rich snippets go, you should basically avoid marking up content that is in now way visible to users and marking up irrelevant or misleading content, such as fake reviews or content unrelated to the focus of the particular page. If you're selling products, reviews for each page, for example, should be about that page's particular product, and not for the store itself.
Read the whole Rich Snippets section here.