Well, don't know if I can bring some new points to bear here, seeing so many other reviews already written on the subject, but I'll give 'er a shot.
What struck me about this game was its versatility. It's like a novel (A very good novel mind you) to which you can choose not only your own ending, but your own plot, and you can shape the characters to fit said plot. I'll elaborate if I may.
Throughout the game, you can play any way you want. You can be stealthy, walking by the enemy leaving them without a clue. You can be adventurous, seraching every nook and cranny and leaving the enemy riddled with bullets. You can walk the grey area in between: (and here's what I like most about it...) Laying intricate traps and ambushes to clear out entire areas before you explore.
The possibilities are endless.
It really does affect the plot, too, when you think about it. You can play as "Snake: Incredible badass action hero.", or as "Snake: Incredibly stealthy modern-ninja". It really changes how you look at the game.
Definately a plus.
This game was just so freaking fun. I've played it over and over on several difficulties, with different tactics, and it has never become boring. I recommend this game up and down to anyone the M rating will allow. Like I said, it can be an action game, a stealth game, an adventure game, an epic game...whatever you can think up.
Whoever you are, this game is for you. Unless you hate good games.