Valve took a lot of heat for announcing the release of Left 4 Dead 2 a mere year after its predecessor, which seems odd considering Madden games get a lot less and Call of Duty games get nearly zero criticism for doing the exact same thing year after year. Many seem to feel that Left 4 Dead was not given a promised amount of support or DLC, or that the content of Left 4 Dead 2 should have only been released as DLC for the original.
Left 4 Dead 2 in a lot of ways is the same game as Left 4 Dead, however it does bring a ton of new content that I think "merely" distributing it as a downloadable add-on would not have been feasible. (Honestly, if it were possible, they probably would've made a lot more money partitioning it out as DLC; so the argument that they were trying to milk customers by packaging it as a separate full-price game doesn't hold water.)
Left 4 Dead 2 brings four new survivors into the zombie apocalypse. The setting is in the southeastern United States, and the survivors have a distinct Southern flavor to them. The levels are a lot wider, meaning the survivors aren't guided down a narrow channel through the level; and the levels occur in broad daylight as well as the middle of the night. There are a lot more weapons, rather than just three different weapons in two different powers. There are also new assists (adrenaline shots and defibrillator units), and all-new melee weapons. There are also new special infected — the charger, spitter, and jockey — and new "uncommon" common infected, like CDA agents in hazmat suits and police officers in body armor.
The Director, the program that controls the overall experience, has some new tools in its arsenal, as well. It has more options for distributing medpacks and weapons (no longer placing the same groups of the same weapons in the same locations), the ability to block off some pathways and open up others to change the overall layout of the level, and even the ability to control the weather in some situations.
Ultimately, though, the gameplay is pretty much the same as before. You have to get through the level from one safe room to the next, trying not to get killed along the way. A couple of the new campaigns end in the same type of "hold out for rescue" event as before, but some have very new objectives. One, you have to collect gas cans and fuel up a car to make an escape (while hordes of zombies try to prevent you from doing it), and another you have to keep moving and run across a long bridge. There is even a new style of "crescendo" event — not only do you have mid-campaign moments like Left 4 Dead where you have to stand your ground against an extended horde for a predetermined period of time (e.g., while a noisy elevator slides into position to grant you passage), but Left 4 Dead 2 has new moments where you start the event at one point and have to battle your way through the zombies to the "off switch" to stop the event, and the horde will only keep coming until you do.
Although each of the five campaigns in Left 4 Dead 2 are considered a "game" and can be played in any order, the dialog at the beginning and end of each tells a more coherent story played in sequence, showing the survivors' journey from Savannah to New Orleans and their ultimate rescue by the armed forces. The game therefore does give just a little more of a story than Left 4 Dead — although there is still no real depth or detail given to the backstory. You still don't know a lot about the characters themselves (unless you read the little bio given in the manual), and nothing more about the infection itself.
Compared to Left 4 Dead, the characters in Left 4 Dead 2 don't show a lot of personality. (The notable exception is that Ellis, if you linger in the safe room for a moment at the start of each chapter, will launch into a very long story about the misadventures of his friend Keith, before getting cut off by one of the other survivors.) While the survivors in Left 4 Dead were constantly throwing out one-liners and brief little interchanges throughout their adventure (from Francis's constant murmurings of things he hates, to the always-entertaining elevator dialog in No Mercy, to Zoey, upon seeing the graffiti "GOD IS DEAD", whispering an awed, "Oh, no, the zombies ate God!"), the Left 4 Dead 2 foursome seem to be all business. They swear a lot more, too, which I don't terribly appreciate.
One thing I noticed about the level design is, because the Left 4 Dead 2 levels are so much more "open", it is much easier to feel lost. When playing the original Left 4 Dead, the level design was so constricted and the use of lighting was so dramatic, that it was quite nearly like riding on rails. There was almost no question where to go next. By contrast, I've found that Left 4 Dead 2 does not have the same "guided" feeling to the design, such that I'm not always sure where to go next. Part of that, I think, is because a lot of the levels are daylight, where "follow the light" just isn't possible — but even in the nighttime of the Hard Rain campaign, the lighting cues just aren't as obvious. In fact, I got thoroughly lost wandering through the sugar mill of Hard Rain, and if I didn't happen to be in a party chat with someone who happened to remember enough of that level to give me some general direction, I'm not sure how long I would've wandered around that level. One of the tricks the game uses to help you find out where to go is, when your character is about to go through the right door or up the correct ramp, he'll signal to his fellow survivors with a helpful "Let's go this way" — but that only seems to occur if you're already heading the right way to begin with. If you can't see or find the right door, you don't get a lot of help. Certainly, this is a problem that will fade with experience, but it does ramp up the learning curve in trying to get through the campaigns.
There are five new campaigns, which are all playable in co-op and versus modes. The Survival mode, that was added as free DLC to Left 4 Dead, is included in Left 4 Dead 2. There is also a brand new "Scavenge" mode, where survivors try to keep a generator fueled as long as they can while the infected try to prevent it. The variety of game types do give you more of an option of how much time you need to commit. A campaign can last up to an hour on the easiest level; versus can be a very lengthy proposition, depending on the skill of both teams. On the other hand, Survival and Scavenge offer a co-op and versus game type, respectively, that generally lasts a relatively short time, like 15 minutes or so.
All told, it definitely feels like much more of a complete package than the original, which by comparison seems like a Left 4 Dead Lite. Although it seems more difficult getting a team together (since its release has been eclipsed by this fall's mega-blockbuster Modern Warfare 2 — or is it because people aren't as excited about this release as the last one?), the teamwork and camaraderie in banding together mowing down hordes of zombies to get to the next rescue point is just as fun as the last time.