One of the best and most enjoyable survival zombie FPS you'll ever play.
I'm basing this review through the console version on Steam. If something I says doesn't apply to the regular PC version, that's why.
+++Good Points+++
+Excellent multiplayer
+New weapons including the new gory melee weapons
+Source engine still looks great
+Controls are very easy and comfortable
+Good use of co-op gameplay
+A lot of replay value
---Bad Points---
-Even with all the new weapons, still feels like there could be more
-AI is horrible and makes single player not fun to play
-No real story
Left 4 Dead 2, released a year after the critically acclaimed first one. So what did you get in this sequel? Well, you got a new cast of characters who are just as good as the first one's, you got new weapons including the awesome melee weapons, new game modes, new special infected, and you get 6 all new campaigns. Now, people may be wondering if they should get this even though they have the first because the gameplay is so similar. You can read this review more and see the detailed answer, or you can stop here and take this short one word answer. The answer is: YES.
The gameplay is one of the highlights of the game for me. You have your standard Valve controls ASDW for your movement, your mouse to shoot and melee them with your gun, and various other keys for jumping, crouching, switching guns, etc.. What makes the gameplay so nice? Well, it's very easy to pick up and IMO it's so much easier to aim with a mouse than this or FPS in general on the 360. The melee weapons, which are the new addition, are very fun to play with but don't just serve a gory new toy to play with. They're very useful because its easy to clear out swarms of zombies around you a melee weapon than with a rifle, pistol, and sometimes a shotgun. Reasons why are because they have no reload, and they make a nice wide swing that can take down numerous zombies in one swipe. Overall, the gameplay is very smooth, and is very nice.
Graphics and audio are a huge plus too. The great thing about the audio is the voice acting is excellent and the characters have nice conversations that have some comic relief to lighten the mood in a zombie apocalypse. The sound effects of zombies falling or screaming is chilling and sounds great. Now graphics IMO are very good too. People complain saying the Source engine is outdated, and while maybe it is, I still think it looks great and really helps get the job done visually. People who complain about the graphics are just shallow idiots who only look at the visuals and not the gameplay. And don't worry, the graphics are more than fine if you worry about them.
The story of the game is very simple, it's just about the 4 survivors having to travel south to avoid the infected zombies. It's very simple, and there aren't any real cut scenes. This is a little disappointing though because a few cut scenes to tie in the campaigns would've been nice, but I guess it's the gameplay that does the talking for L4D2.
Some of the highlights of the game are the excellent multiplayer and co-op experience it provides. No game to this day is better played with friends than this game is. Smash TV and Zombies Ate My Neighbors came close to taking that title, but mostly because those games are fun alone as well unlike L4D2 became the deciding factor. Basically, you can get incapacitated or a special infected can get you (such as a Hunter pouncing on you and scratching at you on the ground or a Charger running into you and slamming you onto the ground repeatedly). Because of this, you MUST stick together as a team because if one person goes off by his own, he's bound to die quickly. Losing just 1 player is enough to make you fail. Having 4 people is almost a necessity because just 3 people is difficult to pull off. That is why this game is so fun with people, because it's great to communicate with friends or players to win (especially in versus mode). Valve is a master of making games be forced to be played as a team, and it's evident in this game and also Team Fortress 2 if you've played that (another excellent game I'll review soon).
Even with all the good things about this game, there's a few flaws. First off, this is more of a personal opinion, but there's not enough guns for me. Yes you have 5 rifles (3 if you don't include snipers), 4 shotguns, 2 pistols, and about 10 melees, but that really isn't a lot. Yes I understand this is a survival game and not CoD, but I think about 5 more guns would've been nice. Also, this game is terrible to play single player or with AI because they play so dumb and never move forward by themselves and only follow you. Now this isn't too big of a deal because this game is so much better with real players, but it's annoying when someone leaves your game and you're forced to deal with a bot (this is most annoying in versus mode). All in all, the games flaws don't hold it back from being a great game, but they are noticeable enough to annoy you.
***Conclusion***
L4D2 is a zombie fan's dream. It's a fun and addictive game that's best played with friends that not only makes it fun to play, but fun to play socially with friends as you yell at each other for wasting the last molotov. It's highly replayable, and with the free DLC from Valve, there's always hope they can add new features such as the new Sacrifice campaign and the re-release of No Mercy campaign from the first game. You'll have loads of hours playing L4D2 solo online or with friends. If you don't want to pick it up from Steam for $20, you can get it for your 360 (if you have one) which is just as good but it doesn't contain free DLC like Steam does. Overall, a very fun game, and worth every penny for it.