Its refreshing to see other game developers on here, but its mostly been us programmers. Design is so critical. A lot of great thoughts in your posts so far.
I'm looking forward to the prototyping phase. Both when I was in AAA and now as indie / hobby we use the paper simulation method, getting a group together, putting game elements on paper and playing through it on a table, discussing what we like, don't like and what could make it better along the way.
But I don't want to steal your fire, so I'll stop there. :) I can't wait to read about that and perhaps other prototyping methods you have used!
RE: Game Design 101- Step 3: Brainstorm