Has the shell of greatness, but the innerds of a failure.
First, is the graphics. They were pretty good, but their so-called "fog of war" effect, did exactly that, made everything look foggy and ugly. The gun models in your hands looked really nice though. The art in this game was it's selling point, the only reason anyone bought the game was because the bad guys looked so awesome, and the commercials made it look like this awesome battle for humanity, but once you get the game you find that the battle seems relatively half-assed.
Which leads me to the gameplay. Your character moves very slowly, unless that is you hold down the sprint button which you can only do for about two seconds before you get tired, and sprinting always makes you lower your gun. The weapon selection is bland. There are two versions of the assault rifle, there's a very difficult to use sniper rifle, a rocket launcher, a grenade launcher, and pistol, a heavy machine gun... basically no weapons that you have never seen in other first-person shooters.
The sound is pretty bad too, as the enemies make loud, boisterous, and annoying battle cries almost constantly. The weapon sounds are OK at best. The audio was just broken.
All in all, this game could have been a truckload better. Basically, if they remade the game from the ground up and made it play like Halo or Half-Life 2, then this game would have been marvelous. I almost can't wait to see Killzone 2 to see if they have fixed any of the awful things that were found in Killzone 1.