My pick would be having features/aspects in the game that don't have any effect on gameplay. This is a problem in some strategy games that try to offer many paths to for the player to win but some paths are so tedious/un-fun/less-efficient-than-others that they end up never used. There are also "features" that basically do not exist. Supreme Rulers are great games in many ways, but the player can research many technologies that lower pollution levels. The thing is that the game doesn't model pollution in any way.