Let's get this Left 4 Dead boycott group stuff out of the way right off the bat. There's one particularly good reason why Valve can get a Left 4 Dead sequel onto the market a year after the original, and that's the game's AI Director. For those of you who haven't played the first game, this AI Director is able to procedurally generate swathes of the 'infected' enemies for players to co-operatively fight against. It can also tailor the infected's attack waves to the skill level of specific teams - the more successful you are, the more infected will be thrown at you. This forms the basis of L4D's gameplay, which is then set across a variety of 'campaign' maps (L4D had four maps, L4D2 will have five).