In 2010, Valve launched a new feature for Steam called 'recommendations', and based on its name alone, I'm sure it's not difficult to figure out its basic purpose. When on a game's store page, you were given the option to click "Recommend" and then write some short thoughts on it. Then, down the road, should a friend check the game out, they'd see your review, which could influence their decision.
Since the feature's launch, over 7 million recommendations have been written - admittedly, I consider that to be lackluster given the sheer number of people on Steam and the number of games available. I'm betting that Valve agrees, and that's the reason this full-fledged 'Steam Reviews' feature has been rolled-out.
Valve calls Steam Reviews an evolution of the recommendations system, because while your friends are still going to be able to see your reviews when visiting a game page, the entire world could see it as well, based on the settings you choose. Further, other enhancements have been included, like the ability to give a thumbs-up or thumbs-down to a game, as well as set the review's language.
With this new implementation, I'd anticipate that the number of reviews written will skyrocket. The fact that the world could see your review rather than just your limited friends list is going to be appealing to many. If you happen to write a review that people like, they can give you a thumbs-up and help you rise to the top of the pile, making it easier for your review to be seen.
At the moment, Steam Reviews is being considered a beta, so it could be that further features will be rolled-out before the official launch. Also, I should state that it feels like a beta at the moment, as changing all of my reviews to "Public" didn't click at first, and even though they are all set to public now, going to one of the affected game pages will tell me that the review is set to "friends only". I am sure that will get fixed up soon, though, and the wait will be worth it given the use this feature will have.