When you go and think about creating a game, you try to define the experience and visualize things in your mind. There is a scenery which you try to achieve through the development process, a scenery in which you want to make the player… sense the experience you designed. You design all the components of the game pointing in this direction. There is a reason for each.. thing in the game to exist, a reason given by you. Everything that you put in the game together form that experience.
From what I understand, this first “Oh my! That’s totally gonna be so cool!” design, is somewhat personal. People usually fail to deliver this enthusiasm to people around them, so they get the game done by themselves. Or if they are not alone, they get to decide on important matters. The “other” team members are of course contributing to the project, but mostly by doing the tasks given to them.