Just a really good FPS
Well as most would admit, the detail of COD4 is pretty good. Though I gave it a point less is because as good as it may look, it isn't really ground breaking. They made a not so unique idea (modern combat) and up'ed the detail by just a bit but not a whole lot. But the big thing for me about COD4 is the fact that it is playable and still looks a little bit good even on Medium Settings (turned off: Specular Map, Shadows, Blur, and Glow for that matter) it doesn't look like clay. Take a look at Bioshock, at high settings it looks oh so good, but then put it on medium and a lot of detail has been removed.
Sound: 9 / 10
With today it is almost impossible to go wrong with sounds. It is relatively easier to capture the sound of an M249 gunshot than produce the 3D model required for it. I ain't really a fan of ambient music but for the whole single player, the sound was pretty much alright.
Gameplay: 10 / 10
A classic fps style gameplay. Though it is interesting that game makers are moving away from the health bar and just going on to visually present low, medium and good health. It is actually harder to make a computed risk if you don't have an exact value of the life of the character. Though this may sound bad, it is actually a pretty good system as it emphasizes that you have one life, and take care of it! They're not the first on a lot of things here such as the throwing back of the grenade, self-regenerating life, no health bars and sprinting. Though they were able to mix it up into a good single and multiplayer experience.
Story: 9/10
This is the first fps that has a story that takes multiple points of view that actually worked for me. It is interesting to follow the different story paths of the two characters but still remain focused on the single goal of the two. There is actually enough diversity to create missions with different goals and paths to achieve them. It is also successful in putting you in the shoes and the eyes of what marines and special forces go through, though the accuracy of which may be debatable but it's still pretty good as a game. One downside is that I felt that the story was pretty short, as I was able to finish it in a span of around 2 days of non-continous play.
Overall:
A nice fps game overall. One of the better games that give you enough reason to move away from the aging multiplayer games such as CS:Source. Also it packs a good enough storyline that would keep you hooked until it is finished.