There are many games I could have listed, such as:
1. Halo 3 and how the autoaim and community ruined the franchise (actually halo 2, but w/e)
2. Unreal Tournament 3 and how Gears of War was able badly influence such an awesome game.
3. GTAIV and how Rockstar took all fun activities and instead incorporated annoying chores.

However, in the end Call of duty 4 takes the cake. It is basically Call of duty 2 in a modern day theme, making it more of the same. That doesn't make it horrible, but including a reward system for people that are already top score in the form of air strikes and helicopters. WORST-DESIGN-CHOICE-EVER!