Well, to me, many FPS's fall into similar environmental categories. Land of the Brown, Land of the Dark Colors, Land of Futuristic-Bright-Lights-o-Rama, or Land of Dirty Neutrals. That I can ignore...kinda.
Then the guns. Many of them, particularly modern realistic, have similar guns. That, I can accept.
Then there are the game modes, and types of multiplayer. Most of the same.
Then the Players. Most of the same, but so,so many of them are undeniably infuriating. Those that Hack, those that Whine, those that Rage, those that Glorify their own movements.
All that I can deal with. Paying for Game 1, 1a, 1b, 1c? ONE TINY SWITCH OR ADD ON DOES NOT FEEL WORTH MY TIME. Nor does higher graphics, better sound effects, or extra maps.
Team Fortress, and other games that have their own unique feel and gameplay, make for a much more worthwhile experience. FPS games that involve stronger strategy, are more worth my time. Those with unique weaponry and combination of weaponry, are more worth my time.
Why tread the same ground unless you are going to actually make something far more inspiring or interesting?
THAT'S why I cannot stand CoD games.