It's really the only visible drawback in Killzone 2 , and despite any erroneous reports you may have heard, Guerilla has no plans to implement a co-op campaign.
Although most realize that this feature won't be included in the game, multiple sources have hinted at the possibility of an upcoming patch that would deliver co-op play for the hotly anticipated shooter. But thanks to a PSU interview with game director Mathijs de Jonge, the rumors have been dispelled and we have the final word: no, it's just not coming. Said de Jonge-
"Currently there are no plans for co-op in Killzone 2. Technically it would be quite difficult to implement it for the story mode as for example the mission sections are loaded in seamlessly no loading screens , and that would cause issues with teleporting players. Even without co-op its still a big game and gamers will have plenty to sink their teeth into."
Yeah, one has to realize that KZ2 isn't exactly like other games, and we get the feeling that Guerilla doesn't want to do anything that might cause the overall quality of Killzone 2 to drop. The big question is, would you rather have a somewhat inferior co-op mode, or would you rather it not exist in the first place? The bottom line is that the single-player mode will be plenty epic, and the online multiplayer should satisfy even the most hardcore FPS fan. You needn't worry about that. Just don't expect co-op, 'cuz KZ2 ain't gonna have it. You can't get everything , you know.
Related Game(s): Killzone 2