(what we know so far)

Doug (developer) has said that first aid kits will not always be available in safe rooms. For instance, it appears that health kits are not in the first safe room in The Parish. Placing them outside (e.g. in ambulances) is meant to encourage more strategic play. Chet has mentioned that Survivors may be able to “double up” on incapacitated teammates to revive them faster.

Special Infected

* New Feature: Some witches walk around instead of crying and cutting themselves.
* New Feature: Gibbing (read the common infected feature) also affects Special Infected on some degree. Survivors can determine how hurt the tank is just by looking at its condition.
* Changes to L4D SI: Some SI we know and love will look different. Hunter, for example, wont wear hoodie - as they are less common in the warm south.

The Charger



He is a big tank looking fella. He charges down an ally, picks up the first Survivor he comes in contact with - uses him/her as a shield as he rushes through the rest of them knocking them back. After that he proceeds to beating the heck out of the survivor he picked up.

Some sources say the Charger has 500 HP, but based on gameplay footage, he may be able to absorb up to 1500 HP from the front (on Normal), possibly due to location-specific damage mitigation (armor). He is immune to bashing. While his charge can hit multiple Survivors, his slamming attack does not appear to have any AOE or cleaving effects. The Charger is primarily designed to disperse Survivors and defeat camping tactics, and as such, he’s weaker against Survivors who are spread out. He also has trouble turning (and possibly stopping), and it may be possible to dodge him or trick him into falling off a precipice.

The Spitter



The Spitter who is able to lob gobs of acid at Survivors. The green projectiles, which appear to have a range similar to a Smoker’s maximum range, create a mist of acid that does damage to Survivors over time. The main purpose of the Spitter is to force Survivors to move out of tight spaces or force them to take damage when they are unable to move (e.g. resuscitating downed Survivors). Little is known about the Spitter, but she will be playable in Versus. The spit attack might take some time to fully deploy, similar to a Boomer’s bile. If you shoot her she'll explode and spit that acid in a large ring around her.
The spit itself is green and luminescent, and when it hits a flat surface, it spreads into a yellow cloud that soon turns to a deep red. The colors may indicate how much damage per second the cloud can do. Based on some videos, the cloud can do a significant amount of damage, perhaps 5 damage per second at the beginning and even more later.

[media]http://www.youtube.com/watch?v=2FyNc57Fsks[/media]
Video of Spitter and Charger attacking.

The Jockey

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The Jockey is a smaller zombie who jumps on the backs of Survivors and tears them apart from behind. Not only that, but he gets to control the movements of the Survivor while he's doing it.
Perfect for separating survivors, luring survivors into traps such as metal detectors and alarm cars and leading them off a ledge to certain doom!
Jockey doesn't have FULL control of the survivor. If the survivor wants to go back and jockey pushes forward Jockey will move slower to that direction. If Surivor press right and Jockey press forward the survivor will head to their front right and so on.

Video of Jockey, Spitter and Charger in action.

L4D ships with 3 new special infected bosses, all playable in Versus and in the "not yet announced" new multiplayer game mode (reveled in October).

Uncommon Common Infected

* New feature: Common infected have more sophisticated hitboxes. You can shoot zombies in the stomach and reveal their bowels, shoot them on the top of the head to reveal brains, neck to reveal the spine etc... Shoot their arm off and they still keep coming at you!
* New feature: Pipebombs don't turn zombies into liquid anymore. They gib just as if you had shot them. Some of them remain whole, some turn into gory gibs.

Hazmat Zombies



"Hazmat" zombies are unique to The Parish and wear environmental suits that make them immune to all incendiary attacks (e.g. Molotovs and incendiary bullets). They can be white, yellow, or green. These zombies were formerly CEDA agents involved in the containment and evacuation operation, but the suits failed to protect them from the infection for some reason. Chet (developer) has said that there will be one area “full” of them.

Mud Men



The second revealed UCI type is Mud Men, who are unique to Swamp Fever. They crawl around on all fours, usually preferring to hide in the waist-deep murk of the swamps. This makes them extremely difficult to spot when traversing the mire. When Mud Men hit players, they cause the players’ vision to be obscured by mud, resulting in an effect similar to Boomer bile.

Bulletproof Zombies



The Parish will have a second type of UCI who wear kevlar and riot gear, making them bulletproof from the front. Apparently, these Infected were part of a private security force who tried to defend against the undead onslaught but found themselves overwhelmed while still wearing their equipment. They will be susceptible to bashing and any attacks from behind, but we don’t know if they are susceptible to melee attacks, explosives, or incendiary ammo (from the front).

Clowns



Uncommon Common Infected unique to the Dark Carneval. Its special ability is to send more zombies come when he attacks.
You can also honk its nose.

In this gametrailers interview Chet says that they haven't showed us all the uncommon common infected, so more will be coming.