Everyone's a critic. No seriously, pretty much everyone is inherently critical of at least a few things. Now, very, very, very few people are actually good critics, but that's an entirely different discussion for another time. Point is, user review systems are now built into everything from Amazon to the walls of most public bathrooms (kinda), so Steam is following suit. Soon, you'll be able to write your own game reviews, vote on other users' all-caps bellowed missives, and follow along with folks whose tastes sync up with yours. The system is now in beta. More below, though after having written this post for only a couple minutes, I'm already prepared to declare it the worst of all time NEGATIVE THIRTY SEVEN BANANA APPLESAUCE OUT OF TEN.
On the upside, Steam's user reviews don't come with arbitrary scores, opting instead for a simple thumbs up/down approach. Also, users can't review games they haven't at least in some way used in conjunction with Steam. This doesn't necessarily mean you need to have purchased a game through Steam, but if it's never been on your list - whether via an activation, Steam key, free weekend, or family share - then no dice.
Beyond that, it's a fairly standard setup. You can vote your favorite reviews to the top of their respective games' pages, track authors, and report reviews that you deem to be spammy or abusive. If developers choose to dive into the crowded critical morass and respond, their comments will be highlighted accordingly. Metacritic scores, meanwhile, aren't being replaced. Rather, user review data will be displayed alongside them.
Now, I do see some potential problems issues here. People are quite fond of using report systems to shut down opinions that aren't necessarily spammy or mean-spirited, but instead are simply opposed to their own. Valve's gonna have to do a lot of up-close-and-personal monitoring and communication to keep this system in check, and that's, er, never really been their thing, to put it lightly. Good user reviews are a handy additional resource, but poorly moderated communities just treat them as a new source of fuel for their flame wars. It's tricky to maintain a balance between those two ends of the spectrum.
Best of luck to Valve on that, but even though the system's only in beta right now, I'm already prepared to give it SEVENTEEN SEVERED APE THUMBS DOWN OUT OF THAT ONE SUPER UNDERRATED SCENE FROM THE NEW THOR MOVIE (YOU KNOW THE ONE).