You can't talk about World War Z (the game) without Left 4 Dead entering the conversation. The team over at Valve and Turtle Rock essentially perfected the zombie survival shooter genre as we know it today. Saber Interactive has basically done another Left 4 Dead, even down to the types of special infected used. The major difference is that this is played from a third-person perspective, not first.
The problem with World War Z's special infected is that they can be almost impossible to distinguish from the mess of standard zombies attacking at the same time. Turtle Rock made a point to have there Hunter, Boomer and Smokers stand out with unique sound design and silhouettes, easily picked out amongst the horde.
The Bull and Creeper blend into a crowd to frustrating lengths and at times feels like a cheap get to almost giving the player no time to react. For example, Creepers will be spawned into the level behind the player or around a corner almost ensuring they will pounce successfully. In my own playtime, this happened more times then I could count on both hands with even instances of Creepers spawning in plain sight. Having them pop into existence isn't fun, and when playing with a team that just isn't cooperating can lead to many game overs.
Then there are the Screamers which can ruin the flow of a session and basically function to draw the player off the main path. Which as an idea is fine, but they never really move from the point they spawned on and well as they're name implies, they don't shut up till they are found and put down. Variety is good, but not at the cost of the fun factor.
Also, it must be pointed out that Hazmat and Bull types may function similarly to that of the Boomer and Charger, but their design is so bland they come to look extremely similar to some of L4D's second tier special infect. Some more imaginative design could've really been used here.