How can a game that involves flying platform things and a fictional planet be realistic? Let me explain: while Killzone 2 is set in a sci-fi universe, the actual gameplay is some of the most realistic of any shooter. Guns are heavy, giving your character momentum and making it harder to quickly move and aim. Firing from the hip is inaccurate, making it necessary to slow down and look down your sight. To escape being ripped to shreds by gunfire, cover is necessary. However, stay behind that cover too long and enemies will lob grenades behind to force you out.
If you haven't played the game, all of this probably sounds really good to you. However, I'm convinced that if Killzone 2 went with a less realistic approach, the game would be much more enjoyable. There's a point where a game becomes so realistic that it's not fun, and Killzone 2 comes dangerously close to that point.