If you receive lots of comments with just a smiley? or a “thanks” etc. Minimum Comment Length plugin let you set a threshold for a comment with a minimum of 15 characters or whatever you like? Once intalled you get a new option page with 2 input fields, in the first of which you can set the amount of characters people should use at minimum for their comment to be accepted. It defaults to 15. If people then leave a comment of less than 15 characters, WordPress dies on them with the message “Error: Your comment is too short. Please try to say something useful.”, or whatever other message you decided to give to them, as this is the second option you can specify in the backend.