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Like the original, Left 4 Dead 2 is set during the aftermath of an apocalyptic pandemic, and focuses on four survivors fighting against hordes of the infected. The survivors must fight their way through levels, interspersed with safe houses that act as checkpoints, with the goal of reaching a rescue vehicle at the campaign's finale. The gameplay is procedurally altered by an artificial intelligence engine dubbed the "Director" that monitors the players' performance and adjust the scenario to provide a dynamic challenge. Several new features have been introduced: new types of infected, melee weapons, and a story-arc that connects the game's five campaigns together.
When it was announced, it received a combination of positive, negative, critical, and community reactions. It also attracted an unusually high volume of pre-release controversy about the game's graphic content. In response, alterations were made to the cover art and the Australian Office of Film and Literature Classification refused to rate the unmodified edition.

Like its predecessor, Left 4 Dead 2 is a first-person shooter with a heavy emphasis on cooperative gameplay. The game presents six new campaigns, each composed of three to five smaller maps. In the first two to four maps of any campaign, the survivors attempt to reach a safehouse, while the final stage requires the survivors to call for rescue and either survive a prolonged onslaught until rescue arrives, pass through an especially challenging gauntlet of infected to reach a rescue vehicle, or collect and deploy fuel cans to facilitate an escape.
Each survivor can carry one main weapon, and either a melee weapon, or single/dual pistols as a secondary weapon. Though melee weapons cause extra damage to the infected when struck, the survivors can use any other weapon or item for weaker melee attacks and to temporarily push the infected back. Players also carry a flashlight, but may startle the infected. They may also carry a single first aid kit or defibrillator, Ammo packs, Pain pills or adrenaline. They may also carry a single throwable weapon — Molotov cocktails to set an area on fire, pipe bombs, and "Boomer" bile, which can be thrown at infected to cause them to turn on each other.
Because many of the special infected can quickly finish off a survivor if they are not rescued, the game encourages players to stay and work together to traverse the level safely. To enable situational awareness of other players, players are shown the health and status of their fellow survivors. If a player does not have direct sight of another survivor, they will be shown the survivor's highlighted silhouette. As they take more damage from zombies, players move slower. If a survivor takes too much damage, they become incapacitated, and can only fight off the infected using pistols until rescued by another survivor. If a character dies, they remain dead until the next level, unless revived by a defibrillator, or, in Campaign mode, reappear in a "rescue closet" to be freed by the other survivors. Should all the survivors be killed or incapacitated, or if all the human-controlled survivors die, the game is over, and players must restart that level.

Left 4 Dead 2 retains the three game modes of the original game—Campaign, Versus, and Survival—and adds a new game mode called Scavenge. In Campaign mode up to four human players fight against the computer-controlled infected to make their way between safe houses; any survivor not controlled by a human player is controlled by the computer. In Versus mode up to four other human players take control of the various Special Infected who try to prevent the Survivors from reaching the next safe house. The two teams swap sides once per
chapter and are scored based on their play through as Survivors. Versus mode typically requires at least 1 human player on each team. Survival mode is a timed challenge where the survivors are trapped in a section of the campaign maps, and try to survive as long as possible against an unending onslaught of infected. In the new four-on-four Scavenge mode, the survivor players are required to collect and use as many of the sixteen gas canisters scattered about a level to maintain fuel in power generators, while the infected players attempt to stop them.

The game also features a "Realism" mode, which can be enabled at any difficulty for either campaign or versus modes. The realism mode removes some of the video game aspects from the gameplay: survivors are not able to see the silhouettes of their teammates; if they die, they can only be revived with a defibrillator kit and will not respawn later in the level.Damage dealt to Infected is also changed, with headshots dealing more damage to enemies, rather than limb or body shots, making gameplay even more of a challenge. Designed to force players to work closely together and rely on voice communication, the realism mode was created to give players a way "to be challenged as a team" without having to increase the difficulty level of the game.

A new game mode called "Mutation" was added as part of the first major add-on for the game, "The Passing". This mode is dynamic and will be updated weekly by Valve to feature new game play ideas. The first week featured a "Realism Versus" mode, which combined the features of the "Realism" and "Versus" modes."Realism Versus" mode proved popular with players leading Valve to introduce it as a permanent gameplay mode.