Itch.io already have a review system, but it's visible only to developers (I believe it's to avoid abuse from competitor devs). Actually it's the developers who benefit (or not) the most from reviews in my honest opinion, not the customers.
I had a terrible experience on another store, which I told past month on "My Life as an Indie Dev" I'm publishing on my personal blog. A competitor dev published a negative review (which also affected my sales during the release) while promoting his own game on this same review (Spam). My game was in second place of the popular new releases, then it left the top popular releases thanks to these dishonest exploits.
I understand that reviews are useful for buyers to make their choice and also believe that it's not everyone who exploit this, but hey, why do we have game demos and refunds then? A customer can't decide for him/herself if said game is for him/her or not? Besides, it's not because someone's else disliked a game that everyone should dislike it as well. Mega Man Legends and Chrono Cross are my most beloved games, but have a considerable hater base.