Someone told me that a designer's job is about making decisions. He wasn't talking about game design specifically, but it seems to fit. In my little bit of experience I would like to add one other thing: designer's fix problems before they happen.
One of the problems we faced with the design of Project Fixbot was limiting player movement. There are times in a level when you need to narrow where a player can go. We do this to help the player find the right path. In a normal platformer this is easy because you just throw a wall at the player and he can't go through. But in our game the player can move on the walls. He can attach to any surface. He moves by launching himself around the map. How can you limit a player that has free rein to any surface?
One of the ways we solved the issue was to limit the angle that the player can launch from. This allows us to steer the player in the right direction. We can now guide the player towards objectives without worrying that he is going to get lost.
So that's my job. I'm Zak, and I'm the game designer. I solve problems.
Reposted from Fixbot Blog: http://demergostudios.com/fixbot/