The issue of "stars", "diamonds", and other kinds of ratings is not really Hilton-specific, as it is a third party who assigns those ratings. Here are the guidelines for AAA ratings:

The Diamond Rating Criteria

Diamond Rating Guidelines

Four and Five Diamond Service Expectations

Rating Process

Member Complaint Procedures

AAA closely monitors the number and type of comments we receive from members regarding listed establishments. When members write to us expressing dissatisfaction with a particular lodging, it is tracked through the AAA National Office Member Relations department. All complaints are carefully reviewed for validity. The establishment is then notified of each complaint so they have an opportunity to respond and resolve the matter within a reasonable period of time.