Left 4 Dead 2 is a multiplayer shooter where you fight through hordes of zombies on your way to safety. It has a few different game modes, but the main focus is on versus mode, in which 2 teams of 4 switch off playing zombies and survivors. The gameplay between both sides creates a lot of variety when playing.
For survivors, their general gameplay is about killing the NPC zombie hordes and gathering items and weapons to help make their journey easier. The biggest, and usually only, threat to the survivors is when the enemy infected team attacks. This is often a coordinated attack in which the survivors need to kill both NPC zombies and special zombies played by the enemy team. If they survive the attack, they continue on their journey, repeating this same gameplay loop until they make it to the end or die.
For the infected team, they take control of several different types of zombies called special infected. Each one has their own specialties and weaknesses, requiring team coordination to succeed. Common gameplay tactics involve setting up ambushes at parts of the map where survivors might need to split up temporarily or are in an awkward position.
The game doesn’t look great by modern standards, but for a ~10 year old game, it looks fine. Because it focuses so much on multiplayer, graphics don’t really affect your enjoyment of the game. The sound design is somewhat inconsistent, due to some great voice acting for both humans and zombies, but some lackluster weapon sounds that can get a bit dull, although this might be due to the lack of weapon variety, rather than weak sound effects. The soundtrack is fantastic and adds a lot of tension and excitement to an already high-strung game.
Left 4 Dead 2 has a decent amount of story, but at least for me, it took a backseat to the gameplay. It can be interesting at times, and certain campaigns focus more on storytelling than others, but in general, plot lines are mostly used to get the main characters from point A to point B.
Overall, the gameplay of Left 4 Dead 2 is extremely well designed and balanced. It is just as fun playing as either the survivors or the infected. It has some minor issues with presentation and story, but they won’t matter to the average player. The only big negative would be the age of the game, but the player base is still fairly large compared to the vast majority of games of the same age.