Product Listing Ads (PLA) will now feature product ratings, starting in late July, the announcement said:
We believe these ratings will help differentiate products across google.com and google.com/shopping and will help merchants drive more qualified traffic through Product Listing Ads. In initial tests, product ratings also helped increase click-through-rates of Product Listing Ads.
The announcement is calling the period between July and October a "grace period" by which "we’ll allow product ratings to be shown across all Product Listing Ads where we have product review data available. After the grace period, we’ll only show ratings for products from merchants who choose to share their reviews with us."
Google Merchant help files explain that reviews are "compiled from multiple sources including merchants, third-party aggregators, editorial sites and users." The approved list of third-party aggregators includes:
Right now, availability is only in the U.S., but "we’ll introduce product ratings in additional countries throughout 2014," help files say.