An emotional journey unlike any other (spoiler-free review)
Let me make this review short and to the point, cutting out all the unnecessary fluff - not unlike Journey itself, a game which certainly does not overstay its welcome, but instead provides an unforgettable experience that fits perfectly together from beginning till end.
I do not wish to go into specifics about the controls, level layouts etc. as I believe the discovery of the game mechanics is a big part of the experience. Or at least it was for me: I went into the game not knowing what it played like at all, and I cannot help but think that this contributed to my overall impression.
So let me instead try to explain how Journey provokes emotional responses from the player - what it does to you, and how.
It is first and foremost a game that appeals to the senses. This is not just because of the wonderful visuals and music judged on their own, but because of the underlying design philosophy, which seems to be about the interplay between the player and the game's (frankly stunning) environments.
You'll move gracefully through these environments in a way that makes you feel as part of them, or as if the environments are a seamless part of you. They may be relatively empty, but - perhaps because of that emptiness - they also feel inclusive. They are not just there for you to drift through on your journey, but the whole point of it - not as obstacles as such, but as the principal 'character' in the game.
Let me try to explain this: It is never a matter of knowing exactly what your goal is in the environment. It is about the discovery within that environment. Many games (most, really) use their environment as a backdrop for foregrounded, reaction-based (or strategic or logic) actions, whereas Journey does the opposite by making the player actions seem like an effect caused by the environment itself.
There may be 'gamey' situations and objectives, but they are always obscured by the foregrounding of the environment, something which is also very important in the game's wordless, but wonderful story, which is told, more than anything, through the use of the environment.
The journey, then, is not about a person traveling through an environment, but about what the environment does to that person (or persons, if you're playing co-operatively online). What physical and emotional responses it evokes.
At times you might be running and soaring joyfully through the dusty dunes. At other times you might just stand there and gaze in wonder. Or you might begin to fear the environment, even as you are still fascinated by it.
Whatever the feeling, ranging from everything to joy and affection to melancholy and sorrow, the point is that Journey is something as rare as a game that plays not as 'entertainment', but as something that resonates within you because it reminds you of life itself.
Further describing exactly what makes Journey so special is not easy, at least without spoiling any part of the story it tells (and offering an interpretation thereof), and it is not easily comparable to many games I can think of (a certain French animated film, Gwen, le livre de sable, springs to mind, though). That is also why I can tell you this: Journey left an impression on me unlike any other game before it.
Play it, and I hope it will do the same to you.