The Stanley Parable is a tough game to review, because it defies so many of the conventions that we've come to expect from any video game no matter the genre. More than that though, it plays with those expectations, especially regarding how choices are presented and made in gaming.
There is a narrator that accompanies you throughout the game, but he doesn't just set up the story. He comments on the decisions you make, taunts you, outright insults you at times, because he disagrees or is utterly baffled by your choice. At it's best you feel like you're getting a live reaction from the developer himself, watching you, reacting to your actions. Right then you start to get the frustration that creators must feel sometimes when something they put out, is interpreted in a way they didn't want or anticipate.
So why not give it a 10? Because how much you like it is going to depend largely on how sick you are of the more mainstream, traditional titles and on how familiar you are with the conventions that the Stanley Parable plays off of. It is quite like a spoof movie in that respect. No matter how well it's done, you have to have seen the original to really get it.