Another smash hit from Valve.
From a first person shooter perspective, this game (and its predecessor) might possibly be the most intense FPS games ever made. They are not as scary as others, though they do accomplish quite a bit of that. The main thing is just in how intense the action feels.
The game is set in post-zombie-apocalypse Earth, as you probably already predicted. You play through the eyes of one of four characters. They have their own unique personality, but besides that play identically to one another. The campaign mode of the game takes you through five series of interconnected levels that lead to a "rescue level". The last mission being the toughest to survive.
The main group of zombies you'll run into are called "common infected", or your average, run of the mill zombie. These are accented by the "special infected" (and also the "uncommon infected", those these don't play a huge role), which make things... much more interesting... Each of the special infected have their own skill that can make your life hell. Several of them can pin you, forcing you to have a friend to help you get out of their grasp.
The campaign levels are a basic path from point A to point B, but the areas are huge, and usually provide multiple pathing options for reaching your goals. Each campaign provides EXTREMELY different challenges, and even the levels within them vary greatly as far as the skills required to not get your ass kicked.
The game focuses on cooperative gameplay, as even if there aren't four players in the game, the missing characters will be bot controlled. The game also gives players the options to define the difficulty of their experience, by choosing the difficulty of the game, and yes/no to realism mode. The experience will change drastically as you move away from easy and towards expert. Just for example, a common infected deals 1 damage per hit on easy, 3 on normal, 5-10 on advanced, and 20 on expert. You have 100 health.
Left 4 Dead 2 provides some of the most solid FPS action I have ever seen. The guns are unique, interesting, and have a good feel to them. Ducking increases accuracy, and weapons have realistic effects on your enemies. The intensity of the action makes you really feel like you're in a zombie movie.
The game as compared to Left 4 Dead (1):
Overall, I would say that this game continues the legacy of the first game. It is slightly better, but the most important thing here is that it is true to the feeling and purpose of the first game, and does not detract from that.
The game does indeed have four new survivors to play as, though the real focus here is on the FIVE new campaigns that are available. Each campaign is very different, even more different than the variance between those of the first game. Each campaign now experiences changes in time of day and weather effects, so your characters aren't always running around in the dead of night. At first, you might feel this could detract from gameplay, but it really doesn't feel that way after you've played for just a little while.
The other MAJOR difference in the game is the new weapons, of which there are TONS. The list for the first game is: pistol (x2), pump shotgun, Uzi, Auto-shotgun, M-16, Hunting rifle, pipebomb, and molotov. Left 4 Dead 2's list is: 9mm Pistol, Desert Eagle, Silenced Mac-10, Uzi, Chrome Pump Shotgun, Wooden Pump Shotgun, Auto-Shotgun (2 variants), M-16, AK-47, FN Scar, Hunting Rifle, Sniper Rifle, pipebomb, molotov, boomer bile. This doesn't even mention the addition of melee weapons, which include: baseball bat, frying pan, guitar, crowbar, wiffle-bat, katana and chainsaw. The addition to the weapon list is HUGE to say the least. The new firearms provide a very healthy selection for players, giving each different basic archetype several variants, and the melee is a great side of the game as well.
There are also a few new non-main weapon items that are interesting. In addition to pills you can now also find adrenaline, which both heals you and provides some extra speed for a short time. These can come in very hand with tanks and horde rush moments. Defibrillators can be found to bring dead players back to life- these are great on rescue levels where there's no closet to be had once it's started. They occupy the med-pack slot. There are also new "ammo packs" that can provide each survivor with a single clip of either explosive or incendiary rounds. The latter is much more useful than the former. These also occupy the med-pack slot. You can also find laser pointers that provide you with a handy red-dot for your weapon, and ACTUALLY increase the accuracy of the gun - the reticle becomes noticeably smaller.
The campaigns show a direct focus on trying to make each one different from the other, and in trying to give some more variety of situations. I wouldn't say the first game failed at this, but Left 4 Dead 2 definitely feels much less repetitive because of their efforts in this area. Rather than simply "attracting the horde" to get across areas, now you often have to perform another action before they'll stop coming, such as shutting off an alarm, etc.
The game, in my opinion, is a few notches harder than the original. Keep in mind this is coming from someone who ALWAYS played the original on expert, period. As far as I can tell, it's the new additions to the special infected that cause this leap in difficulty. The main factor here being that there are now 6 types to be had, and FOUR of them can "pin" you. The new special infected are the jockey, the charger, and the spitter. The jockey is more annoying than anything, but does require aid to dislodge him from your shoulders. If left alone, he can and will kill you, so he can be dangerous. The charger is a kind of a weaker variant of the tank, though his speed and ability to pin you can make him deadly in the right situation. He can also hit all survivors with a single charge, so if you're trying to stick together against a horde rush, you better take him down before he has a chance to mess you up. The spitter is less dangerous because of her lack of direct attack - most of the time. She can't pin you, but she can make trying to stay in the same spot instantly lethal. She spits corrosive ichor on the ground in a huge area. It isn't very damaging if you leave it quickly, but you also can't survive it by trying to stay still. Simply leaping onto an object that's off the ground will keep you safe, but that's not always a choice. Her main annoyance comes in messing up survival plans that require staying together in a small, protected area. Kill her quickly.
The tanks have actually become a _little_ bit easier, as they seemed to have lost some speed. In the original, they could run you down pretty quickly. Here, you can outdistance them with ease, provided you don't back yourself into a corner. The witches can now roam around, but otherwise their mechanics are the same as far as health and damage.
The zombies now have EXCELLENT damage effects. Rather than simply getting bloody, or immediately dying when you shoot a limb off, their bodies are now very destructible. Chunks of flesh and bone will fly off when shot with the more powerful guns, and there are tons of ways they can come apart. They also can run around with a limb gone, so it feels more zombie-movieish in that way.
The difficulties are pretty much the way they were in the original. On rescue missions, however, two tanks seem to come on the second round of tank appearances, instead of just one. Also now available is realism mode. This can be activated on any difficulty, and always has the following effect: items no longer glow with helpful outlines that indicate their presence/location, fellow survivors no longer glow when you look at them with an obstacle in your way, there seems to be less supplies in general, zombies receive damage mainly through head shots (rendering body shots MUCH less effective), the witch is instantly lethal, and there are no rescue rooms for fallen comrades. The only way to bring back dead allies is to make it to the safe room, or use a defib.
Although minorly important, I would note that the campaigns in the game actually link to one another, unlike the seemingly randomness of the different ones in the first game. If played in order, you'll end up in the next campaign via the method of escape from the previous one. Not that the first game was anywhere near ruined by the lack of a plot-link, but now it just makes more sense.
Overall the game is fantastic, but it's ALSO fantastic when compared to the first game. The mechanics are _slightly_ different, but definitely aren't anywhere near a complete switch, and as a stand alone game the is one of the best FPS games ever. The cooperative gameplay makes things so much better than simple head-to-head multiplayer, and the fact that it's an integral part of the game is very impressive and fun.
In other words, go get this game.