Story. The story is fairly simplistic, the Helghan invasion of Vekta. In this you are one of four playable characters that on their own decide to take the fight through the Helghan lines and up to space where they get revenge from the traitor Adams. Although the characters aren't too complex, they are all a variation of a soldier, without being reduced to 2 dimensions. Each character has a backstory, personal motivation and unique play style.
The game progresses in the classical level to level structure that more and more modern games seem to run from and offers a compact tight experience where the bullet counter goes mental. The levels are linear with slight variations depending on the character selected for the current section. Instead of having an open area where the best choice would be dictated by the characters talents, you get a restricted version of the level that forces you to use the talents of that character. That would be a minus but actually works well because when you play with Rico you want a number of enemies to be made available for machine gunning, Luger on the other hand needs to be separated from the group to be able to sneak and do quite suppressed shot or knife takedowns. That said playing the campaign with Luger doesn't protect you from prolonged gunfights where if you don't run out of ammo you are doing it wrong.
The gun play is key in this game. Some people would say that a first person shoter on a console payed with a joypad isn't ideal. That would be true for arena shooters and other fast paced action games. In KILLZONE the joypad works better because it works with the way the controls give you the feeling of stepping on the ground and holding a gun. Mouse and keyboard FPS games make you feel like a floating gun construct hovering over the ground at inhuman speed. Strafing and turning are also limited, the outcome of a firefight does not depend on reflexes. If you want to win an engagement you need a lot of bullets for suppressive fire, location location location and perfect corner clearing habits. You quickly learn that you better be oriented and covered, if you lose the direction of incoming fire and run out of cover you are f-ed. My complaint is that the hard difficulty allows you to take a few more bullets then I would like, and thus diminishes the challenge but that is redeemed by the way that grenades do not forgive, a grenade close enough to you kills you, no joke. And if you run out of cover to avoid it you just get shot. This is why KILLZONE shines even today, once the game is half way you better have learned your lesson because the tolerance for bs is nil. You have to be careful you need to spot the enemies before they spot you, yours should be the first shot fired and you need to fire it when you are in an advantageous position. And if there is a grenade launcher or rocket launcher on the other side you better identify it and take it out first because it's your ass.
The AI in the game blew my mind at the time, enemies know what's up, avoid over aggression, they do eventually charge out of cover but they do it guns blazing so don't you underestimate them. It is more a mechanic of the game than an AI failure, because otherwise some engagements will not end at all.
All in all this is one of the games that really made me wish to go into game development and is one of the games that I will be revisiting for as long as I have access to it. I made a let's play of it available on my youtube channel if you have somehow missed it and would like to see what I'm talking about.