I wrote a brilliant batch of flavor text, I came up with a really good character generation system (where "really good" is defined as: doesn't immediately make me want to spork my eyes out) and now...I'm stuck.

I have an idea for what I want to write about gameplay, I just don't know how to write it.  And then there's the issue that I came up with an idea about something that actually *says* something I want to say...ugh.  I don't know.

I think I'm just whining.

That said, I'm hip deep in a game about dream spirits saving humanity from destroying itself, so maybe it doesn't suck too hard.

Someone was relating an experience of writing a game starting with a description of play.

Perhaps that would work for you? Instead of instructing, just describe what the people in your mind's eye are doing, how they're interacting. And you can have placeholders like "And then they [use dice or cards to figure out who speaks next] and then the next person begins back with the spirits again" or whatever.

Also... maybe put your critic self away for a couple of days. Now's the time to barf out all the stuff, & then pick through for gems. IMHO, anyway.
This is your first draft. Let it suck.

& dream spirits saving humanity from itself are pretty awesome.

I would love to play a dream spirit saving humanity from destroying itself, so I hope you'll un-stuck soon.

If I can have one advice for you this is: If you can't write something at the moment, skip it for now. Do other stuff, which comes easier. Do pauses and think how this easier things correspondes with what you don't know how to write. You don't have to write from the begginig to the end.