As Colin has noted, forum posts can be rated and those ratings can feed into the gradebook. Here's an article I wrote about implementing star ratings in Moodle: https://matbury.com/wordpress/2014/02/implementing-star-ratings-in-moodle/
If it's of any interest to you, I also wrote an article on a research paper that measured some negative effects of peer-rating systems on social media platforms, i.e. comments on news sites, which bear some similarities to typical discussion forum discussion and peer-rating tasks: https://matbury.com/wordpress/2014/05/ratings-systems-on-social-platforms-can-unexpected-effects/
I hope this helps!