Rich Snippets Get Sent To The Corner
In the past Google has been fairly slow in their response and resolution for spammy approaches to organic marketing. This is not one of those cases. It was just last July (2011) when Google, Bing & Yahoo officially agreed to conform to one form of structured data. The adoption of this markup by websites has been gradual but today in many common queries you will find the results on Google to include elements like author profile pictures, star ratings and other elements not previously delivered as part of the normal search result.
Today Google has updated their webmaster guidelines to explain where, at their discretion they will choose to exclude the elements of microdata that could be delivered as rich snippets. Why? If the web page that houses this structured data is deemed as spammy or the use of the structured data itself is considered spammy Google will likely not deliver your result as an enhanced snippet.
Here is what Google has said for guidelines to understanding what is spammy in this scenario:
If your page is about a band, make sure you mark up concerts being performed by that band, not by related bands or bands in the same town.
If you sell products through your site, make sure reviews on each page are about that page's product and not the store itself.
If your site provides song lyrics, make sure reviews are about the quality of the lyrics, not the quality of the song itself.
What Does This Mean For Your Business?
Every website will have reason and place to use structured data and potentially enhance your search results to increase your probability of a higher click-through rate. The best approach in this case is to use microdata only where it has specific purpose. As you do this your website will be more likely to perform better in search engines both in terms of ranking and click-through.