Check the comment for a set minimum length and disapprove it if it's too short. Make your commenters leave meaningful comments!
Don't you find it annoying when someone comments with just a smiley? Or a "thanks"? On quite a few forums you have to leave a comment with a minimum of 15 characters to be able to comment, but WordPress doesn't allow you to set such a threshold. With this plugin, it does. It easily checks the comment for a set minimum length and disapprove it if it's too short (except if you're either an editor or an admin).