Steam is firmly in charge as the main platform for PC games, and within Valve's ecosystem you can, if you so wish, consult user reviews of games before making a new purchase. The system is handy for anyone in doubt of a given game's qualities, but it's also one developers can misuse to artificially push the average score higher than it might perhaps be otherwise.
Naturally this will in turn boost sales, and if these reviews are biased or misrepresented, well, it's not what Valve intended for the system. Therefore they have now decided to make an alteration to the Steam reviews setup, in which people who procured a game through a Steam key will be locked out of reviewing it. In other words, only folks who buy a game through the store will be able to submit a review for it.
As developers seeking to inflate their scores mainly do so by distributing codes to players (in exchange for shining reviews) Valve hopes this small alteration will reduce the problem.
Thanks, Gamespot.