Jump right into the middle of the action!
The single player story is short, I'll get that out of the way now because it's the only little detail where the game lacks utter perfection. The actually story itself is so epic and well crafted though that it really doesn't matter that it's short. You'll spend most of the game as sgt. 'Soap' Mactavish an SAS straight out of selection. The story reflects Soap's status as the FNG and at first you really won't have a whole lot of resposibility on missions, but this gradually builds up and it really feels like you're earning your teammate's trust. The story starts when your SAS team finds a nuke on board a russian cargo ship they've been tasked with raiding. this leads you on a campaign to find out what's going on in the Middle East and Russia that in some ways eerilly mirrors real world events. The story takes many twists and turns and is not your typical 'my country tis of thee' war story, by the end the game actually comes across as very anti-war.
The game's missions are largely varied, you can be pretty sure you're going to spend a lot of time shooting people in the face, but the missions are executed in a variety of ways. For instance there's a sneaking/sniping mission which is really in contrast to the missions that mirror the invasion of iraq where you're storming down the streets with cars and grenades blowing up around you.
Speaking of which this game is immersive, it gets you as close to the action as you should ever want quite frankly. There's been a few times where I've been so absorbed I've felt genuine fear for my life. I've tried to actually dodge the bullets coming towards me as if they might hit me through the TV screen and things to that effect. Sounds are so realistic, there's gunfire, explosions, your teammates are almost always talking (shouting in the case of the marines, over all the exploding) and the enemies also like to shout at you.
Weapons are genuinely very nice in this game and that's really what a shooter's all about. The guns are all realistic (no BFGs, boo), some have laser sights which help with aiming, some have iron sights... which don't. You'll most likely find your own favorite weapon combos, every gun handles at least a little bit differntly. This really comes into play in the multiplayer mode where you can select your weapon loadout before you begin, it really helps you play the game more comfortably.
The multiplayer is probably the most exciting and well put together portions of this game. The system is based on a persistant character who you level up throughout matches. Leveling up earns you more guns and perks, along with more of the slightly complex game modes. Don't think it's bad to be a lower level though, a level 1 can still easilly murder a level 55, it's just more flexable when you reach the higher levels, you get more options. Matches are always fun and are a big challenge (it is kind of funny to see your friend get killed about 8 times in a row as he respawns) but even if you're new you'll have fun and it really does feel like a big achivement to win a game.
Overall Call of Duty is an excellent package and one of those few games out there that may actually be worth shelling out $60 to own. Completing the singleplayer earns you some extras when you're done and the higher difficulties are stupidly challenging, enough to keep you coming back for more if you're as stubbourn as me. It's really a game easilly deserving of it's high praise, a game sure to be the gem of your collection.