Want to help other gamers make informed purchasing decisions? Valve is giving you an expanded voice on Steam to leave reviews, good or bad, of any game you've played. Simply called Steam Reviews, the beta launch of this new initiative is an evolution of the Recommendation system that has been a part of Steam since 2010. If you're written Recommendations before, you can make them visible as Reviews by changing the appropriate setting in your user profile.
"Users considering a purchase on Steam may now browse reviews from friends and the community, leave comments on reviews, and indicate which reviews they found helpful," Valve explains. "Once a user has played a game or used a software title on Steam, they are able to write a review of that product and share their experience with everyone on Steam."
The reviews don't create an overall score or rating for a particular game at this time, though Valve is looking to implement that feature sometime during the beta. Also, it's not necessary to have purchased a game through Steam to leave a review, though you do have to launch it via Steam.
Developers are able to respond to reviews, though they can't delete a user review, at least not directly. They can report reviews they find abusive or offense, as can other users, which will draw the attention of a moderator to make a final decision.