I've been on RP sites that gave you points/currency when you finished threads, based on things like word count and other factors.
I don't mind admitting that I gamed the hell out of it along with some other people who were essentialsly my accomplices. We would plan out multi-thread plots, figure out ways to have more things to write about, and post multiple times per day, often in excess of 2-3k words per post, all to get that sweet sweet currency.
The interesting thing is that it DID make us really active participants. We were excited to pull in new players and show them the ropes (currency bonus, new RP partner, etc), RP activity was very high, etc. I eventually burned out for various reasons, but I was certainly motivated until then.
The rewards were varied- more power for your character (narrative power, mostly- that didn't really have stats or anything), special races, in character followers, higher positions in the factions, etc.
We are currently pretty loose with all of that and I don't think it's a bad thing. I'm not sure how I feel about people 'buying' factions/etc- is that a healthy way to expand the setting? I see Wes' point that it's good for the site to have activity.
I'm also concerned about the maintenance of this system. Who does it fall on to make sure points are tallied up and distributed? Who makes sure that the posts aren't low effort lorem ipsum blocks? How do we make sure that Wes' wallet can handle the influx of everyone going for commissions (I'm just cheap enough to zero in on that as a goal!)?
While I have my reservations, I do like the idea. I up voted the suggestion. I think that we need to think very critically about what behaviors we want to encourage/discourage and the cost of those decisions while implementing this.