Someone who thinks that game development involves telling a bunch of coders, modelers and writers what to do as if they were a movie director. Usually shows up on various game-related forums and says they have an idea for a game or mod that they need help with. Most don't directly refer to themselves as an Ideas Guy.
Some Dude: Okay, I thought up this AWESOME idea for a game, and I'll need a modeler, mapper, programmer, writer and music composer to pull it off.
Some Other Dude: Lemme guess, you'll be the ideas guy?