Animal-like speed, senses, reflexes and CQC characteristics are staples of the zombie strategy. Survivors have culturally been known for having ranged weaponry with limited ammunition. What makes the zombie concept fun is assaulting a team in hordes and having the agility to dodge gunfire while quickly closing the gap on your targets, striking too swiftly for them to resist or escape. Survivors meanwhile should focus on how to keep zombies from closing said gaps while conserving ammo and sticking together.
MW3's Infection mode gives zombies throwing knives, which breaks the CQC rule and allows them to take out survivors instantly from a respectable distance. This doesn't pan out well because zombies, being disposable in large numbers, can get an angle on the survivors' holdout and start off every life by pot-shotting with throwing knives until they kill someone - which isn't hard to do at all.
There's not much else to say. Most matches I played in MW3's Infection ended with zombies slaughtering survivors each round; it was hardly a contest. The throwing knives were the problem. In fact, zombies would often chuck their knives right from the outset and kill survivors before they could organize themselves on the other side of the map. This is not how Infection should be played. But why should anyone be surprised? The game mode was a blatant afterthought; the rest of the game was recycled garbage from MW2.
L4D2 got the zombie projectile concept down and functional. Smokers and Spitters couldn't snipe survivors because they required CQC to maximize ranged damage.
Leave Infection mode to the developers that know what they're doing; we'll leave the copy-paste garbage with you, IW.
