In September of last year, Valve made changes to Steam's user review system in an effort to combat "artificially inflated" review scores. The problem, as Valve saw it, was that free game keys could be used by developers to manipulate review scores, and so the solution was simple: Reviews based on games that were acquired from outside of Steam—review copies, giveaways, purchased from other retailers—would not count toward a game's final overall rating.
The system was tweaked shortly thereafter so that reviews written by people who received games for free would be visible by default, but the scores themselves would remain locked out of the overall ratings. It was a compromise that allowed players to share their thoughts about a game with those who wanted to read them, without unduly influencing the more numbers-driven consumers who prefer to stick with the straightforward "positive/negative" system when making their purchasing decisions.
Yesterday, Valve tweaked the system further by eliminating any form of freely-received game from its total review score, rather than just game keys that were acquired from non-Steam sources. "The review score (shown at the top of store pages and in various places throughout the store such as search results) will no longer include reviews by users that received the game for free, such as via a gift, or during a free weekend," Valve wrote. "Reviews can still be written by customers that obtained the game in any of these ways, but the review will not count toward the overall review score."
I'm not completely convinced that eliminating opinions is the best way to move toward a more effective scoring system, and I imagine that some developers won't be happy about their free weekends no longer having an effect on their overall ratings (assuming the effect would've been positive). Nonetheless, Valve said the change will "improve the relevance of the score by better reflecting the sentiment expressed by invested, paying customers." Naturally, free and free-to-play games will not be affected.