Search results that are enhanced by review rich results can be extremely helpful when searching for products or services (the scores and/or “stars” you sometimes see alongside search results).
To make them more helpful and meaningful, Google is introducing algorithmic updates to reviews in rich results. This also addresses some of the invalid or misleading implementations webmasters have flagged to them.
While, technically, you can attach review markup to any schema (content) type, for many types displaying star reviews does not add much value for the searcher or website visitors. With their latest change, Google is limiting the pool of schema types that can potentially trigger review rich results in search.
“Self-serving” reviews aren’t allowed for LocalBusiness and Organization
In the past, an entity like a business or an organization could add review markup about themselves to their home page or another page and often cause a review snippet to show for that page. That markup could have been added directly by the entity or embedded through the use of a third-party widget.
Google considers this “self-serving” because the entity itself has chosen to add the markup to its own pages, about its own business or organization.
Self-serving reviews are no longer displayed for businesses and organizations (the LocalBusiness and Organization schema types). For example, Google will no longer display rich review snippets for how people have reviewed a business if those reviews are considered self-serving.
Find out more in the Google FAQ over on the Google Webmasters blog.