Left 4 Dead 2

Left 4 Dead made a name for itself largely with the help of its AI director system. The idea was that the game would track what players were doing, and send in varying levels of zombie swarms and items accordingly. This system was a big success, and gave both L4D and its sequel great replay value. Before its release, The Passing was billed as the first Left 4 Dead level in which the AI director would take control of the story in addition to the intensity of enemy attacks. In an interview on gametrailers.com, it was suggested that characters from the first and second games would meet and develop new relationships dynamically based on the whims of the AI director. While this is true, the payoff is absolutely minimal. There are only two short scenes in which the survivors interact with each other, and while there are a large number of variations on these scenes, few of them are really interesting. It would have been nice to see the original survivors get more face time, although it is interesting to see what has become of some of the most well-liked characters from the original game. 
 
In terms of the actual gameplay, The Passing is the shortest and one of the least interesting of the campaigns. Better than Dead Center but not as good as any of the others, the campaign does include some new enemies and weapons to keep things fresh, and it's still a blast to run through in co-op. The new melee weapons and achievements, in particular, are a lot of fun to explore. It's just a shame that it has to be over so soon, and that the last leg of the level is just another gas hunt similar to the end of Dead Center. 
 
The real selling point of The Passing, and the aspect that seems to have been publicized the least, is the addition of Mutations in the online multiplayer mode. Every week, Valve will be tweaking a classic multiplayer game mode and releasing the result as a "mutation." These modes are only available for a week, until the next mutation shows up. Mutations, while apparently not always fair (the first is Realism Versus mode, in which the survivors have all of the Realism rules enabled, giving the zombies a big advantage) will definitely give L4D2 even longer legs than it originally had. Couple this with the new maps, achievements, and weapon types that come courtesy of the new campaign, and The Passing is a must for anyone who enjoys L4D2's competitive multiplayer.