They're not clownin'
Valve is tired of people treating its reviews system as a joke, so it's doing something about it. For Valve that's pretty uncharacteristic, but hey it's something. In a blog post directly on the service, the company explained that "at least" 160 games have had more positive spins based on the fact that those with a granted CD key provided better reviews.
"Passion" is a qualifier for some of those, Valve states, but "in many cases, the abuse is clear and obvious, such as duplicated and/or generated reviews in large batches." As such, all reviews where a key was provided won't be included in the score -- they'll be listed and you can read them, they just won't be tallied. If developers try to bend the rules, Valve will end their relationship.
Well this is some serious stuff, and an interesting point in the era of undisclosed influencing. There have been many cases with mobile developers giving out hundreds of free keys "as long as you give us a five star review! ::winky face::," as well as the widely known "bot boosting" phenomenon, and this could help cut down on that. At the very least, you can still read a legitimate review from someone who did just get a key through a contest or Kickstarter or similar means.
Though it does suck that, say, all of the Humble Bundle folks won't be able to impact the score -- maybe there'll be a compromise at some point. But Valve isn't very good at that, so some people will probably have to go without it for a while.