In games where the main focus is on multiplayer, many people want to know whether its single-player mode makes the game worth purchasing. As a single-player myself, I often have the same question and so I bought this game mainly for the purpose of being able to write this review and answer that question.
Firstly, I found the game's menu confusing. There are quite a few modes available, but all of them bar the one that says single-player are for playing online. I found this out by clicking on them and finding myself thrust into the middle of an online game. I'm sure the players were just as surprised as I was and after quickly dying, I made a hasty retreat. Nothing in the menu really warns you of this and the particular labels such as realism, survivor, mutation, scavenger, versus etc. are not self-explanatory. There is a way to play some of these modes singly, but it involves entering codes and cheats into the console. I did experiment with this very briefly and experienced some graphical issues, but didn't feel like investing the time in it to see if there was a way to fix this. If you visit forums, you will see all of these terms being discussed and it can be very confusing to the novice.
In regards to the single-player portion, I found this sequel better than its predecessor although both games have their loyal fans and will argue the point. You play as one of a team of four and even with bots for partners, you really get the sense of belonging. In fact, the game is structured in such a way that you must integrate and act with the team or you will die. You must also protect them and they you or you will die. In other words, you must always be aware of being a team. This principle applies in any of the modes. I normally don't care for team games, but this one did a very fine job of it.
Your team is referred to as the survivors and stranded in a hotel with the area swarming with zombies, you must make your way to an extraction point and safety. The campaigns and levels are divided by reaching safe houses along the way where you will find supplies of weapons, ammo and health. There are no saves in between, so you and your team must survive the level. Although the distance between safe houses is fairly short, the intensity forces you to move slowly and so the game might seem longer than it actually is. If you die, the level fails, but if a bot team member dies you can continue playing and are also given a chance to revive than person. You can share health and I can't stress enough how important it is to keep all of the team alive, as your chances of surviving are much lower if somebody dies. Fortunately, the bots are very quick and have keen eyesight and will save your butt often. You must return the favor.
The game is quite challenging, so you will get replay value. The levels are also designed to never play exactly the same way twice, so you still have to keep sharp every time through. If you're into achievements, there's lots to be had, but you'll find the stats somewhat skewed as a single-player as only you will be able to perform certain actions such as operating big guns, using bombs, the chainsaw, the grenade launcher or melee weapons. The bots also like to jump in front of you just as you're pulling the trigger and then chew you out for shooting them.
Although I have plenty of hours in this game, mainly due to restarting levels, the single-player portion really is quite short. There was one level added later and the last campaign is taken from the first Left4Dead and played as the original survivors. It would be nice if they added further single content. However, you can play this game a long time if you opt into multiplayer and this is the first game, so far, that has actually tempted me. There's lots of new content being added there. If you would like to try multiplayer, this would be a good introductory game as the concept is simple and you can focus on improving your skills. I would advise researching this first though, as some of the more serious community can be quite unforgiving of inexperienced players and they like you to have a microphone. Getting vote kicked out of games seems to be a common occurance in some of the modes.
I encountered very little technical problems besides the occasional momentary hitch where the games freezes for a moment. Otherwise, the game looks good and plays smoothly on my system. It's a fun game, but I wouldn't pay a great deal for it unless you go multiplayer.
Something about this game just kept drawing me back to it and so I started doing a little more investigation into mods. I discovered that there were many community made add-on campaigns that could be played as single-player. I have so far added and played about 30 of these getting super value from the game. Valve has also added an official community made campaign in beta form called Cold Stream and announced that it will be porting the original Left 4 Dead maps into Left 4 Dead 2.
Left 4 Dead 2 now includes all Left 4 Dead maps. However Left 4 Dead can still be purchased and also has many community made mods.