1) Generate a BTC address for every user, who has posted at least N comments in last X months.
3) Every time you mod a comment or article, coins are transferred
Comments and suggestions are welcome.
Paying using a quantitative metric and not qualitative metric will likely not give the results you are looking for. Especially if a bot can profit from it.
As far as a forum which acts as a wallet as well, Ogrr does this.
Witcoin used to do this.
It's quite a task to get users to pay:
Witcoin couldn't figure out how.
CoinSmack was along the same lines:
There have been calls from Reddit /r/bitcoin to add these features (e.g., tipping). Reddit is open source so it may happen someday.
Quora has a Quora credits incentive system that has had mixed reviews.
There should be a winning approach where micropayments can help build and foster community and participation, but it has been elusive so far. I see success from the crowdsource microlabor task platforms (TaskRabbit, Zaarly, etc.) but those are a "do this and get X" type of concept. When it goes lower to Q&A types of sites (e.g., Quora, Reddit) or Forums, people won't pay for service nor tip. At least that's that what I'm seeing.