I wouldn't even say that the the campaign of Killzone 3 was better than Killzone 2. The encounter design was pretty bad and call of duty ish, and they never really had confidence in anything new that they were doing, like jetpacks et all. You're left with 'oh no a bunch more enemies spawned directly in front of me, time to shoot shit up!'.
The story was fairly bad, on top of it all off, and that's a huge deal to me. Even though Killzone 2 dragged somewhat as a single player campaign, the mood of it all was very successful, almost too much, considering the hate that Rico got. Killzone 2 also had far better set pieces and visuals even if the graphics engine is more dated than what was used in Killzone 3's campaign. Finally. as much as Killzone 2 only sparingly made use of 'happy accidents (things like the gas chambers flying about), these things were completely abandoned in Killzone 3, which made the whole thing feel a lot less realistic and visceral and more like a video game.
As much as this is just a ramble of complaints, not only did Killzone 3 dumb down the story and characters and damage the hinted at complexity of the universe (no, name dropping accords here and there and giving us a comically simple 'closer' look at helghan society doesn't count), the ending also fucked up the entire universe. The sheer amount of contrivances that they're going to have to jump through to even make a Killzone 4 is going to be something, and if they're keeping their new writer... man.