Comments to articles and forum posts often result in a mix of a good and useless posts. It is very easy for good posts to be lost to other posts. A digg style ability to bump a a good post and push it to the top of the heap would go a long way in keep users more engaged in reading and responding to comments especially with article comments. I dont think something like a 5 star rating system would work well for something like this. See http://digg.com/ Some other forums have a similar function "Was this post helpful? Yes or No" Basically if it's good it gets a bump other it doesn't.