As has been mentioned before, KZ2 was the exception not the rule.
Also, I feel KZ2 is pretty overrated on the internet. The game has its fair share of flaws that it gets a pass on because of "oooh pretty graphics".
Some of the flaws of KZ2, just in my opinion: the gunplay isn't up to par with the other AAA shooters on the market, the game is fairly shallow, the story is bad, the characters are generally unlikable, the game is very linear, the difficulty is all over the place.
I did enjoy the little multiplayer I played, but that brings up another point, I'm primarily a singleplayer guy, so all these flaws are magnified for me.
I certainly wouldn't miss GG or the Killzone series if either just magically vanished one day.