I've noticed it takes a while for people to realise that a Scoop site allows them to submit stories and discuss them.
I described the basics of how Scoop works to my neighbour the other day, and he was quite surprised (in a good way) and said that it's a unique idea for basing a site around - which would certainly explain why a lot of users don't really "get it" at first.
So yeah, I think you just have to work the culture of your site (what there is of one, in a new site) and really push the whole "submit stories and comment too" thing.
Forcing a certain ratio will require code changes though. And more selects :-p
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Discuss the art and craft of writing