1. A method performed by data processing apparatus, the method comprising: obtaining, for a product or service, quality feedback including multiple different quality scores representing different measures of quality that have been assigned to the product or service by multiple different users, the quality feedback including feedback text submitted by the multiple different users with the quality scores; identifying, from the feedback text, a characteristic, of the product or service, that has been referenced in the feedback text submitted by the multiple different users; determining, by a data processing apparatus, an amount of influence of the characteristic on the quality scores that were submitted with the feedback text based on a difference between a first aggregate value of the quality scores that were submitted without feedback text identifying the characteristic relative to a second aggregate value of the quality scores that were submitted with feedback text identifying the characteristic, wherein the amount of influence corresponds to differences in quality scores that are attributable to the characteristic; identifying, for a given user, a quality profile specifying a characteristic weight that is a measure of importance of the characteristic to the given user; determining, by a data processing apparatus and for the given user, a personalized estimated quality value for the product or service, including adjusting an overall quality score for the product or service based on the influence of the characteristic on the quality scores based on the characteristic weight for the given user; and ranking, by a data processing apparatus and for the given user, the product or service among other products or services based on the personalized estimated quality score.