It seems (and I myself am partially guilty as well) that many people were hoping (even though they knew better), that COD 4 would be more realistic, at least via the PC version; something similar to old R6, Ghost Recon, and possibly even as ruthless as SWAT 4. But really, we were just kidding ourselves.
We should have known that it was COD2 more or less, and that a ranking system with points scoring was destined to give it a SOCOM feel.
If you were looking for something more realistic, consider these obvious flaws that they didn't address, that we've seen in COD2 and other 'shooter' games. It may make you reconsider...
1. Insta-prone: This is when you can run around and drop straight to a prone no matter where you are or what you are doing, to not only gain accuracy quickly, but to avoid taking damage, as well as confusing your enemy(throwing off his aim). This was a big problem in COD2 and BF2 (though they fixed it in BF2). Basically when someone does this to you, you should just laugh, because they're essentially saying "I suck, I had to do it or else you would have killed me!"
2. Jump shooting : This is something that you take with a grain of salt. In self-promoted fragfest games like UT, Quake and etc, this is very common and accepted. There's no qualms about the process. Yet in games like CounterStrike, COD2 and COD4, it was an aggrivating maneuver that gained you movement and evasion advantages, while allowing you to still be a threat to your enemy in terms of outgoing fire/shooting. As previously mentioned, we accept it openly in known fragfest games, but it's extremely out of place in any product that is centered around anything relating to historical warfare. Could you imagine a civil war game with guys jump shooting musket guns? Then trying to 'Jump Reload' them? You'd have to have slow down bullet time effects just to stay in the air long enough to reload those ancient weapons.
3. Turbo straffing: Again, like with Jump Shooting, Turbo Straffing is openly accepted in fragfest games; titles where most of the avatars or characters are super human in size and strength and endurance, or are armored with a special body suit that increases their abilities.
Yet when it comes to anything in the theater of realism, it's sorely out of place. I don't know about you, but shooting an assault rifle is pretty hard. Shooting an assault rifle while moving is exceptionally hard. Shooting an assault rifle while moving back and forth, side to side non stop is a ridiculous notion in terms of both accuracy and stamina. To give yourself an idea of how ludicrous it is: Stand up from your computer, back up a few feet and give yourself some space. Don't wear anything heavy(sweaters, jackets, shoes etc)or have anything in your hands. Now, attempt to straffe like you do in the game, moving back and forth left and right constantly. Notice how awkward that feels? Can you see yourself tiring out soon? Stop, and while you get your breath, think about how difficult it would be to do that with thirty+ lbs. of military equipment and gear on, while under fire, while trying to retain even a SLIGHT bit of accuracy....WHILE on the move...??? Think you can still do it? Think you might even throw a jump move or insta-prone in there too?
4. Hip Shooting: The game rewards you more for using open 'hip shooting' rather than iron sight aiming. This was to be expected though, and it won't change for these types of games. Obviously a sniper rifle will require a scope most often, but an assault rifle or pistol would, unfortunatley, be better used without the sights in this game. Either the accuracy for non sights is too high, or it's too low for sight mode(s).
5. Throw back grenades: Yes, it COULD happen, but most likely only in the movies. When you see a grenade you run like a son of a bitch; you don't pull out some cinematic move and grab it like Rambo and try to throw it back.
6. MP engine: It's built the same as COD2. All of the online and network connectivity woes of COD2 have reared their ugly head with COD4, thanks to poor master server designs, limited connectivity options and the lovely punk buster module.
Naturally, these aren't a big deal to most players, because A) They like how COD2/4 plays and B) They haven't a clue what realism is, and nor do the developers, so it works out quite well.
With that said, if you just accept that it's not realistic, and doesn't seem that it was intended to be, you can swallow it's faults a bit easier and focus on what it is good for.
Which unfortunatley isn't much when it comes to MP. If you liked COD2 and you like modern warfare, then this is probably your deal though and worth a purchase.
If you don't, for whatever various reasons, then you probably won't spend much time playing it, SP or MP. The game is fun, no doubt, but it's 'par' for MP, and a tad over average for SP, which as we all can agree is considerably short and means I wouldn't expect anyone to pay more than twenty bucks for it. Would you pay more than twenty dollars for a mod or an expansion; because that's pretty much what COD4 is.
Sound 6
Graphics 8
Presentation 8.5
Gameplay 8 (6 native, but another two points for the cinematic style action)
Replay value SP: 5 MP 8
. All my claymores were used up in the first wave. So basically im stuck. 
. Anyone know any good sniping spots?
