The modern setting for this superb Call of Duty sequel is so deep and well-developed thanks to its story and then more.
The single-player is short, with the length half as long as most first-person shooters like Half-Life 2 or Halo 3. What makes up for the short single-player is the story. You play 2 characters, one from S.A.S. (British), and the U.S. Marines. There's other supporting characters, one of them you get to play in a mission set years before the actual story-line. The story is better and consistent, you truly feel you're in a battle of a great story, and not playing someone who's in a famous WW2 battle. From start to finish, you'll never know what will happen next and who knows you'll survive. It's an adrenaline-pumped thrill ride borrowing elements from 24 or The Kingdom involving action and politics, and this game does so well. One of the best missions is where you only have a pistol and a sniper rifle along with only 1 team-mate. You and your team-mate have to go through the entire mission without alerting people, you could but you'll be outnumbered. It's a chilling experience and it makes you really feel like you're doing some tactical.
There's not a lot of replayability to the single-player unless you want to play it on the hardest difficulty, or Arcade Mode which is interesting like Halo 3's.
The multi-player is where Infinity Ward focused and man is it good. You could create your own character not by his appearance but by his set of weapons, and perks/abilities. All the weapons are fairly balanced to say the least but the perks are sort of... off. It'll take more to kill someone with a "Juggarnaut" perk (extra health), grenade launchers, and one perk you might hate is "Martydom" which is where a person who dies, in a second a live grenade explodes in the location of his/her dead body. It's annoying and it ruins the fun. Shanking is a pretty cool element combing stealth.
With that said, the multi-player does have problems. It tests your patience hard and asking too much. You have to pay attention to everywhere you go, which I like, it gives you a great feeling of intensity. But there are instances where you die for an unfair reason even if there's a KIll-Cam. There's also really bad spawns, where you die instantly from campers, enemies, airstrikes, helicoptors, grenades, turrets and other stuff that's going on. So much stuff going on isn't a good thing all the time. With that said, the multi-player isn't that bad and its quite enjoyable, but please consider the problems I've listed, you've been warned.
The graphics are the best, in instances in the single-player you'll see the range from a sniping point to the rest of the town, and it's beautiful. The multi-player is toned down a bit, but still looks really impressive. The sound is the best, you hear gun sounds far away or next to, characters screaming, grenades, cars blowing up, helicoptors and airstrikes makes you feel like you're in war. The tilt is good, and the first-person view changes if anything happens to you. If stunned, you move slower. If you need cover fast, the corners of the screen have blood vessels shown here. If you fall off a big height, you hear a cracking sound and a black blink on the screen. It's great to see Infinity Ward paid attention to little things like that.
Overall, Call of Duty 4 is without the doubt the best of the series, the single-player might not last long but it's so deep you'll love how it goes from start to finish even if... well see yourself. On top of that is great multi-player which actually is enjoyable at spots, but rarely you see you and your friends just having a private match and not going online. If you're a COD fan get it, and if you're into fast and tactical shooters, you should get this as well. War is hell but this game is heaven.