Excellent graphics, great story, great gameplay...what more do you want?
The game has excellent graphics, among the best on the PS 2. The character effects are very realistic, from movement to aiming and reloading. I don't like the limited weapon inventory (they should go back to the Goldeneye 007 who carries everything he finds, I mean it's just a game, right?).
The enemies, called Hellghast, are very cool. They look like a cross between Colonial Marines and Robocop, and are insidiously clever. they take cover, run from grenades, shrug off wounds and talk to each other. As you play through the game you find different, tougher Hellghast.
The selection of weapons is very solid. The usual fare is represented (rocket launcher, sniping rifle) to bolt-action grenade pistol and much more. You can also employ a variety of gun emplacements such as a grenade launcher or heavy MG. The damage physics are realistic, and if you're a good shot, you can take out the Hellghast quickly.
The story is good too. You're in the middle of an interstellar war against the Hellghast, mutated colonists who've had enough of your imperialistic ways. They won't go down without a fight though.
There are only two drawbacks to this game, as I see it. One, the difficulty of the game is rather high. There's very little room for error, even on the easiest setting. It's frustrating, sometimes. Why have different skill settings if they're all impossible?
Two, there isn't much in the way of multiplayer. It's mostly online; I guess game programmers don't realize that the vast majority of gamers are not involved with online games, contrary to what the video game media state.
Otherwise, Killzone is probably one of the best FPS on the PS 2.