Users having a different title based on their number of posts/comments
E.g. instead of just saying "Posts: 22" it might say next to it, "Has the potential to be special" or reaching 500 posts/comments gives you a "Legend" tag line next to it
Different taglines for reaching certain goals of comments/posts
Not only does it reward users who comment more, it also improves the trustworthiness of a user and what they are saying.