Every one of us who are involved in the roleplaying hobby are game designers. Sure, it comes in matters of degree - some players never get much more involved than creating their characters, while on the other end are the obvious ones who tackle creating entire games. But even sitting down and planning out a session is game design.
That's a great hippy-feely notion, but it begets a corollary. It's easy to get lost in the "design mode" and just come up with something that's not fun or doesn't work quite right in actual play (as I firmly believe happened with Exalted 2e - looked good on paper, but not so great in practice for me). It's also easy to get caught up in "gamer ADD" and change things in the middle of things just because some new method, technique, house rule, whatever caught your interest.
|That's my designer hat, right there|