Where to being with this one... oh boy.
Storyline- Call of Duty 4 is the first Call of Duty game to not be featured in the World War 2 era. Call of Duty 4 takes place in a fictional conflict set in the middle east and Russia. It's pretty much the same old war-torn story, but it has alot of depth. You really grow with the characters, and you get some emotions bumps and bruises when things take a turn for the worst.
Graphics- Inifinty Ward (creators of Call of Duty) have outdone themselves this time. The graphics are crisp and pop out of the screen. The lighting effects are great, as well as the different effects that indicate damage. The weather effects in this game are phenomenal as well.
Audio/Music- Honestly, without the superb audio that plays throughout the game, Call of Duty 4 wouldn't be as good of an experience. It not only plays like a real war, but it sounds like one too. You won't be hearing any rock song in the backround, only gunshots, and pandemonium. You can hear the whizzing of sniper bullets, the sounds of falling debris after an explosion, etc etc.
Playability- Not only does the game sound and look amazing, it handles great too. Not once will you find yourself feeling like you're playing a game. The controller will feel like an extension to your hands, and once you get used to it, you'll barely notice that you are pressing buttons at all.
Parent's Guide- I highly recommend that you do not let anyone under the age of 14 play Call of Duty 4. It's not only incredibly violent, but it's just way too realistic for a child to play. Some things shouldn't be exposed to children and pre-teens, and one of those things is the cruelty of war.
So what's the verdict? Buy it. Don't wait... buy it now. If there is any game that you pick up within the next month, make it Call of Duty 4, if you haven't already.