The original Killzone was meant to be Sony's "Halo killer," a phrase that was being thrown around a lot back when Microsoft's flagship franchise ruled the roost and nothing else could compete—you know, before Call of Duty took over.
But back then Killzone was just a flashy but ultimately unfulfilling shooter on the PS2, a platform not known for great games in the genre. Luckily that changed with Killzone 2.
The jump in platform generation to the PlayStation 3 helped, but ultimately it was a host of smart design changes that made Killzone 2 so much better than the earlier title. Everything looked better and the gameplay had far more weight and realism; it was so impressive at E3 2005, when it was announced, that gamers hardly believed they were seeing a real video game.