The single player is amazing, it is an open world environment which isn't normally done for a FPS. You start out in a jeep with this guy driving you to a hotel in the middle of this country where 2 factions are at war with each other and you've come to kill one man "the Jackal". This guy has been supplying both sides with weapons and ammunition.
There is a lot of detail in this game, for one when you get shot you actually have to take a knife and pry out the bullet before you can heal yourself (online too), which makes you use cover to regain health. When you throw a grenade and it explodes it kicks grass up and knocks leaves off of trees. If your car starts to smoke, you have to get out, pop the hood, and use a wrench to fix the engine. When you pick up a gun your hand actually reaches down and picks up the gun. These are some of the little things that make it a great experience.
The best part out of all the little things though is the fire. If you don't want to buy this game, you should at least go rent it, put the game in your 360 and just throw a molotav cocktail in the grass and just watch the fire spread, it is truly amazing.
All of this and I'm leaving out the truly groundbreaking map editor, which you just must see to believe. The ability to make your own map, publish, share, and play it online with your friends is the greatest part of this game.