How about a like/dislike thing ala Facebook? Each LIKE is worth a point and you reach certain levels based on how many points you have. Gay RPG analogy but as the levels go up, they require more points.
This allows you to gain rank for multiple smart comments from multiple people in the same thread, so yes, people will 'rank up' quickly. So reserve the better perks for higher levels. This also will help people moderate each other because assholes like myself who may say something stupid will quickly demote. Maybe have it so that if you demote too quickly, it's an automatic temp-ban?
Yes, the first thing that came to my mind was "well than people will game the system for better and for worse" but then i realized that if people took the time to uprank someone so much, they are losers. If someone downranks quickly, they probably deserve it. I think once the "fad" of it initially wears off, it will slow down (also why having perks awarded for later levels makes sense)
Don't know if it exists (I'm sure some RPG forum has this)