There are various ways to go about doing this but the definite thing that should be avoided is a way to make people annoyed. Thus penalties of any sort should be out of the question as they can be abused.
Some ideas include
* Starred Rating: Similar to missions, people would be able to rate a thread or even a post in a thread in a 5-star rating. Having a low rating would not bring a penalty but having a high one would be useful in perhaps assigning badges or perhaps Dp. For example a 4-star rating would give the poster of that a 2 DP reward while a 5 star rating would give a 4 DP reward.
* Simple vote rating. People would be allowed to vote/digg/like a post which might make it easier again to assign badges or DP to the poster
* Thanks: Allow a way to thank a person, similar to the
Ubuntu Forums . Once again, a great way to assign badges.
And finally, another cheap way to promote forum participation is by providing badges or DPs on their use. Some ideas:
* Badges based on any of the ideas mentioned above
* Assign badges explicitly in the same way
Duels does it, This would not be based on community rating but rather on the whims of the almighty admins