Instead of making a list longer than a dictionary, I'll make a few points.
1) M4A1 takes 3 direct shots to the skull to get a headshot without the magical perk called Stopping Power. What exactly is Stopping Power anyway?
2) Desert Eagle(and almost ever other gun) takes 2 shots to the head to get a headshot with Stopping Power.
3) The UMP45. Why is it stronger than every gun except the Barret? I shouldn't have to hit you 6 times in the chest with an MG4 to kill you, when you can hit me twice in the leg with a UMP45 and kill me.
4) Sprinting reduces registration. When someone is sprintingg in a straight line towards you, the first 5 bullets you fire into them don't register. This has been tested and proven. Even on split screen on consoles, so it isn't a lag problem.
5) EMP produces static on the screen. Are my eyes digital? Why would any part of my body that isn't prosthetic be affected by an EMP?
6) Silenced M9 Pistol(and at least 3 other guns that I know of) still shows on the radar when fired. Really?
7) The hit boxes for the riot shield are horrendous. You can easily be shot or stabbed through the middle of it. There is noticeable turn lag while using the shield, meaning when someone tries to quickly run around you and shoot you, if you turn quickly, they can kill you. If you watch the kill-cam, you won't even be facing them, when on your screen, you were looking directly at them. Activision has acknowledged the problem and has no plans to fix it.
8) Auto-aim that can't be turned off. "Why thank you auto aim, for making me aim at and follow the guy running across my screen that doesn't know I'm there, instead of letting me continue to aim at the stationary one shooting at me. Now neither of them are dead, and I am.". Ever try to shoot an enemy claymore to blow it up as the enemy runs up the stairs next to it? Won't happen. The game think that instead of blowing the person up with their own claymore, you'd be happier shooting at their toes.
I can list many more things wrong with the game that have nothing to do with the players. Let me know if you need more.