And you're so right; I haven't put down to paper everything that's in my head, and there are things that, when I put them down I'm like 'Ugh, why did I write that?' But I'm hoping to have a good layout.
Yep, writing it down also helps you remember what the vision was a few weeks/months down the line. Basically we all have our own ideas of what a game should be like, and especially on MSB even in the most well meaning of ways we'll bombard the fuck out of you telling you what we would want to see; give us enough time and we'll dilute your ideas . Having them all in a document helps you keep things a bit more pure.
I remember arguing with @Coin when Eldritch launched about everything having to be his way or the highway because I was - and still am, naturally - sure what I was suggesting was better. But here's a funny thing - even if it was he was still right (and it hurts me to type it) because that's what makes a game stand out more than anything else; a creator excited to be doing their own thing.
Too many games, IMHO, are missing that spark and just settle for the generic brand name instead.