I go in-depth into my thoughts about a 'game designer' position here.
The entire post is important in my opinion (says the person who wrote it), but here is one snippet amidst many:
I'd love for someone to adequately explain in detail why changing the reload time of the sniper rifle in Halo 3 from 0.5 seconds to 0.7 seconds was beneficial. That is a game designer. Someone who constantly has in the back of his mind that it takes exactly 2.4 seconds for the player to do a complete 180 degree turn around in whatever specific FPS he's working on, and the effect it has on competitive play. This is the science of game design. It should be called "gameplay mechanic engineer", IMO.