Call of Duty's gameplay is just unmatched. No other FPS has managed to capture the same fluid feel. It just feels right, and that's why I like it over other ones. There is no speed up or slow down when you move to aim. Controls are responsive and tight. This game allows me to utilize my reflexes better than most other games, and is very satisfying after a good game. It makes me feel like I was really on top of my game.
Killzone, on the other hand, does everything else perfectly. The graphics are BEAUTIFUL. Everything sounds fabulous, and I love the maps. The Helghast are awesome characters, and I love the diversity they tried to implement. The different classes with diverse abilities were a good idea, but poorly executed. The game is dominated with snipers and their OHKO rifles and invisibility, and the other class (can't remember) that is able to disguise themselves as a teammate. The Killzone series has perfected atmosphere, and Call of Duty has perfected gameplay. Unfortunately, I don't think you could combine the two without detracting from them. The weighty feel of Killzone increases immersiveness, but also takes away from the fluidity.
Uncharted isn't fps
then what is it?
A third person action-adventure game with a competitive multiplayer mode as a bonus.